diff --git a/src/Codegen/TargetUnifier.php b/src/Codegen/TargetUnifier.php
index c13275b..d52ad6f 100644
--- a/src/Codegen/TargetUnifier.php
+++ b/src/Codegen/TargetUnifier.php
@@ -671,7 +671,7 @@ private static function returnTypeBranches(
         ?\ReflectionType $type,
         \ReflectionClass $declaringClass,
     ): array {
-        if ($type === null) {
+        if (!$type instanceof \ReflectionType) {
             return [['mixed']];
         }
 
diff --git a/src/Codegen/TypeRenderer.php b/src/Codegen/TypeRenderer.php
index add6174..e821567 100644
--- a/src/Codegen/TypeRenderer.php
+++ b/src/Codegen/TypeRenderer.php
@@ -33,7 +33,7 @@ private function __construct() {}
      */
     public static function parameterType(?\ReflectionType $type, ?\ReflectionClass $declaringClass = null): string
     {
-        if ($type === null) {
+        if (!$type instanceof \ReflectionType) {
             return '';
         }
 
@@ -83,7 +83,7 @@ public static function acceptsMatcher(?\ReflectionType $type): bool
      */
     public static function returnType(?\ReflectionType $type, ?\ReflectionClass $declaringClass = null): string
     {
-        return $type === null ? 'mixed' : self::resolveParent(self::render($type), $declaringClass);
+        return $type instanceof \ReflectionType ? self::resolveParent(self::render($type), $declaringClass) : 'mixed';
     }
 
     /**
@@ -150,7 +150,7 @@ private static function resolveParent(string $rendered, ?\ReflectionClass $decla
      */
     private static function resolveSelf(string $rendered, ?\ReflectionClass $declaringClass): string
     {
-        if ($declaringClass === null) {
+        if (!$declaringClass instanceof \ReflectionClass) {
             return $rendered;
         }
 
diff --git a/src/Defaults/DefaultFactories.php b/src/Defaults/DefaultFactories.php
index 250db53..7a7f736 100644
--- a/src/Defaults/DefaultFactories.php
+++ b/src/Defaults/DefaultFactories.php
@@ -66,7 +66,7 @@ public function valueFor(string $requested, ?RuntimeContext $owner = null): ?arr
         $owner ??= Runtime::current();
         $factory = $this->factoryFor($requested);
 
-        if ($factory === null) {
+        if (!$factory instanceof \Closure) {
             return null;
         }
 
@@ -79,7 +79,7 @@ public function valueFor(string $requested, ?RuntimeContext $owner = null): ?arr
 
         $valueOwner = is_object($value) ? Runtime::ownerOf($value) : null;
 
-        if ($valueOwner !== null && $valueOwner !== $owner) {
+        if ($valueOwner instanceof \Rasuvaeff\Understudy\Runtime\RuntimeContext && $valueOwner !== $owner) {
             throw ContextOwnershipViolation::forDouble();
         }
 
diff --git a/src/Defaults/TypeDefaultResolver.php b/src/Defaults/TypeDefaultResolver.php
index 7531885..9df147a 100644
--- a/src/Defaults/TypeDefaultResolver.php
+++ b/src/Defaults/TypeDefaultResolver.php
@@ -35,7 +35,7 @@ public static function forSignature(
         bool $nested = false,
         ?object $double = null,
     ): mixed {
-        if ($signature === null) {
+        if (!$signature instanceof \Rasuvaeff\Understudy\Codegen\MethodSignature) {
             return null;
         }
 
diff --git a/src/Expectation/ArgumentFormatter.php b/src/Expectation/ArgumentFormatter.php
index bec8437..e6aea7d 100644
--- a/src/Expectation/ArgumentFormatter.php
+++ b/src/Expectation/ArgumentFormatter.php
@@ -59,7 +59,7 @@ private function __construct() {}
      */
     public static function scope(callable $render): mixed
     {
-        if (self::$aliases !== null) {
+        if (self::$aliases instanceof \SplObjectStorage) {
             return $render();
         }
 
@@ -82,7 +82,7 @@ public static function format(mixed $value, int $depth = 0): string
             return '…';
         }
 
-        if (self::$aliases === null) {
+        if (!self::$aliases instanceof \SplObjectStorage) {
             return self::scope(static fn(): string => self::format($value, $depth));
         }
 
@@ -224,7 +224,7 @@ private static function formatObject(object $value, int $depth): string
     private static function alias(object $value): int
     {
         $table = self::$aliases;
-        \assert($table !== null);
+        \assert($table instanceof \SplObjectStorage);
 
         // `contains()` is deprecated as of 8.5; `offsetExists()` is the same
         // lookup under the name the language kept.
diff --git a/src/Expectation/Expectation.php b/src/Expectation/Expectation.php
index 9be07ee..f5855c0 100644
--- a/src/Expectation/Expectation.php
+++ b/src/Expectation/Expectation.php
@@ -168,7 +168,7 @@ public function hasCaptors(): bool
      */
     public function setGuard(callable $guard, string $description): void
     {
-        if ($this->guard !== null) {
+        if ($this->guard instanceof \Closure) {
             throw InvalidCallSpecification::guardAlreadyNarrowed($this->method, $this->guardDescription);
         }
 
@@ -189,7 +189,7 @@ public function setGuard(callable $guard, string $description): void
      */
     private function acceptsGuard(array $args): bool
     {
-        if ($this->guard === null) {
+        if (!$this->guard instanceof \Closure) {
             return true;
         }
 
@@ -349,7 +349,7 @@ public function matches(string $method, array $args): bool
      */
     public function matchesArguments(array $args): bool
     {
-        if ($this->tailMatcher !== null) {
+        if ($this->tailMatcher instanceof \Rasuvaeff\Understudy\Matcher\TailMatcher) {
             $fixed = $this->argumentCount - 1;
             /** @var int<0, max> $fixed */
 
@@ -517,7 +517,7 @@ public function describeAgainst(array $args): string
      */
     private function describeGuardAgainst(array $args): string
     {
-        if ($this->guard === null) {
+        if (!$this->guard instanceof \Closure) {
             return '';
         }
 
@@ -539,7 +539,7 @@ private function describeGuardAgainst(array $args): string
      */
     private function acceptsGuardDefensively(array $args): bool
     {
-        \assert($this->guard !== null);
+        \assert($this->guard instanceof \Closure);
 
         try {
             return ($this->guard)(...$args);
diff --git a/src/Invocation.php b/src/Invocation.php
index 5cf832c..2d06c5f 100644
--- a/src/Invocation.php
+++ b/src/Invocation.php
@@ -181,7 +181,7 @@ public function recordOutcome(Outcome $outcome): void
      */
     public function recordReturned(mixed $value): void
     {
-        if ($this->returnedState !== null || $this->outcome !== null) {
+        if ($this->returnedState !== null || $this->outcome instanceof \Rasuvaeff\Understudy\Outcome) {
             return;
         }
 
@@ -201,7 +201,7 @@ public function recordReturned(mixed $value): void
      */
     public function recordDiscardedReturn(): void
     {
-        if ($this->returnedState !== null || $this->outcome !== null) {
+        if ($this->returnedState !== null || $this->outcome instanceof \Rasuvaeff\Understudy\Outcome) {
             return;
         }
 
@@ -226,7 +226,7 @@ public function isReturnDiscarded(): bool
      */
     public function recordThrown(\Throwable $thrown): void
     {
-        if ($this->returnedState !== null || $this->outcome !== null) {
+        if ($this->returnedState !== null || $this->outcome instanceof \Rasuvaeff\Understudy\Outcome) {
             return;
         }
 
diff --git a/src/Outcome.php b/src/Outcome.php
index 0aed476..554a82a 100644
--- a/src/Outcome.php
+++ b/src/Outcome.php
@@ -53,7 +53,7 @@ public function didThrow(): bool
     public function returned(string $method): mixed
     {
         if (!$this->returned) {
-            \assert($this->thrown !== null);
+            \assert($this->thrown instanceof \Throwable);
 
             throw OutcomeUnavailable::threwInstead($method, $this->thrown);
         }
diff --git a/src/Runtime/DoubleState.php b/src/Runtime/DoubleState.php
index fd734c8..4b5be1a 100644
--- a/src/Runtime/DoubleState.php
+++ b/src/Runtime/DoubleState.php
@@ -422,7 +422,7 @@ public function declaredExpectations(): array
      */
     public function settle(): void
     {
-        $remaining = static fn(Expectation $expectation): bool => $expectation->cardinality() === null;
+        $remaining = static fn(Expectation $expectation): bool => !$expectation->cardinality() instanceof \Rasuvaeff\Understudy\Cardinality;
 
         $this->expectations = array_values(array_filter($this->expectations, $remaining));
         $this->matchingExpectations = array_values(array_filter($this->matchingExpectations, $remaining));
diff --git a/src/Runtime/Runtime.php b/src/Runtime/Runtime.php
index f58cdaa..9e9117a 100644
--- a/src/Runtime/Runtime.php
+++ b/src/Runtime/Runtime.php
@@ -168,7 +168,7 @@ public static function popScope(): void
     {
         $fiber = \Fiber::getCurrent();
 
-        if ($fiber === null) {
+        if (!$fiber instanceof \Fiber) {
             $context = array_pop(self::$main);
 
             if ($context instanceof RuntimeContext) {
@@ -197,7 +197,7 @@ private static function stack(): array
     {
         $fiber = \Fiber::getCurrent();
 
-        if ($fiber === null) {
+        if (!$fiber instanceof \Fiber) {
             return self::$main;
         }
 
@@ -219,7 +219,7 @@ private static function push(RuntimeContext $context): void
     {
         $fiber = \Fiber::getCurrent();
 
-        if ($fiber === null) {
+        if (!$fiber instanceof \Fiber) {
             self::$main[] = $context;
 
             return;
@@ -267,7 +267,7 @@ private static function unremember(RuntimeContext $context): void
     public static function liveContexts(): array
     {
         $current = self::currentIfAny();
-        $contexts = $current === null ? [] : [$current];
+        $contexts = $current instanceof \Rasuvaeff\Understudy\Runtime\RuntimeContext ? [$current] : [];
 
         foreach (self::$live as $context) {
             if ($context !== $current) {
@@ -299,7 +299,7 @@ public static function adoptClone(object $clone): void
     {
         $blueprint = DoubleFactory::blueprintOfGenerated($clone::class);
 
-        if ($blueprint === null) {
+        if (!$blueprint instanceof \Rasuvaeff\Understudy\Codegen\Blueprint) {
             // Only a generated class reaches this method, and every one of them
             // is registered when it is compiled.
             return;
@@ -453,13 +453,13 @@ public static function forget(object $double): void
 
         // The facade has already answered for a stranger, a retired double and
         // a foreign context through `stateOf()`; what reaches here is owned.
-        \assert($owner !== null && $owner->stateOf($double) !== null);
+        \assert($owner instanceof \Rasuvaeff\Understudy\Runtime\RuntimeContext && $owner->stateOf($double) instanceof \Rasuvaeff\Understudy\Runtime\DoubleState);
 
         $owner->forget($double);
         self::owners()->offsetUnset($double);
         self::forgotten()->offsetSet($double, value: true);
 
-        if (self::$retiredOnPurpose === null) {
+        if (!self::$retiredOnPurpose instanceof \WeakMap) {
             /** @var \WeakMap<object, true> $retired */
             $retired = new \WeakMap();
             self::$retiredOnPurpose = $retired;
@@ -473,7 +473,7 @@ public static function forget(object $double): void
      */
     private static function fibers(): \WeakMap
     {
-        if (self::$fibers === null) {
+        if (!self::$fibers instanceof \WeakMap) {
             /** @var \WeakMap<\Fiber, list<RuntimeContext>> $fibers */
             $fibers = new \WeakMap();
             self::$fibers = $fibers;
@@ -487,7 +487,7 @@ private static function fibers(): \WeakMap
      */
     private static function owners(): \WeakMap
     {
-        if (self::$owners === null) {
+        if (!self::$owners instanceof \WeakMap) {
             /** @var \WeakMap<object, RuntimeContext> $owners */
             $owners = new \WeakMap();
             self::$owners = $owners;
@@ -501,7 +501,7 @@ private static function owners(): \WeakMap
      */
     private static function droppedByScope(): \WeakMap
     {
-        if (self::$droppedByScope === null) {
+        if (!self::$droppedByScope instanceof \WeakMap) {
             /** @var \WeakMap<object, true> $dropped */
             $dropped = new \WeakMap();
             self::$droppedByScope = $dropped;
@@ -515,7 +515,7 @@ private static function droppedByScope(): \WeakMap
      */
     private static function forgotten(): \WeakMap
     {
-        if (self::$forgotten === null) {
+        if (!self::$forgotten instanceof \WeakMap) {
             /** @var \WeakMap<object, true> $forgotten */
             $forgotten = new \WeakMap();
             self::$forgotten = $forgotten;
@@ -533,7 +533,7 @@ public static function isOwnedByCurrentContext(object $double): bool
     {
         $owner = self::ownerOf($double);
 
-        return $owner !== null && $owner === self::current();
+        return $owner instanceof \Rasuvaeff\Understudy\Runtime\RuntimeContext && $owner === self::current();
     }
 
     /**
@@ -586,12 +586,12 @@ public static function dispatch(object $double, string $method, array $args): mi
         $context = $current;
         $state = $current->stateOf($double);
 
-        if ($state === null) {
+        if (!$state instanceof \Rasuvaeff\Understudy\Runtime\DoubleState) {
             $context = self::ownerOf($double) ?? $current;
             $state = $context->stateOf($double);
         }
 
-        if ($state === null) {
+        if (!$state instanceof \Rasuvaeff\Understudy\Runtime\DoubleState) {
             // Returning null here would violate the declared return type of
             // any non-nullable method, so say what actually happened. The
             // message differs for a double retired through `Understudy::forget()`:
@@ -686,7 +686,7 @@ private static function answer(
         $verdict = SequenceVerdict::NotWatched;
         $sequence = $context->armed;
 
-        if ($sequence !== null) {
+        if ($sequence instanceof \Rasuvaeff\Understudy\Runtime\ArmedSequence) {
             $verdict = $sequence->offer($double, $invocation);
 
             if ($verdict === SequenceVerdict::OutOfTurn) {
@@ -744,7 +744,7 @@ private static function answer(
                 self::$inlineFloor = $floor;
             }
 
-            if ($signature !== null && $signature->returnsNever) {
+            if ($signature instanceof \Rasuvaeff\Understudy\Codegen\MethodSignature && $signature->returnsNever) {
                 // The expectation returned instead of throwing; returning from
                 // a `: never` method is a TypeError by language rule, so name
                 // the real mistake rather than leaking that.
@@ -775,7 +775,7 @@ private static function answer(
             return self::forward($state, $double, $method, $args);
         }
 
-        if ($signature !== null && $signature->returnsNever) {
+        if ($signature instanceof \Rasuvaeff\Understudy\Codegen\MethodSignature && $signature->returnsNever) {
             // Which mistake it is depends on whether anything was configured:
             // "you never said what this throws" reads very differently from
             // "nothing expected this call at all".
@@ -788,9 +788,7 @@ private static function answer(
         // configured. Anything else is indistinguishable from a step arriving
         // out of turn — the protocol cannot tell "not part of this" from "you
         // got the order wrong" unless the test says which it is.
-        if (!$matched && $verdict === SequenceVerdict::NotAStep) {
-            \assert($sequence !== null);
-
+        if ($sequence instanceof ArmedSequence && !$matched && $verdict === SequenceVerdict::NotAStep) {
             throw VerificationFailed::of([self::unconfiguredUnderProtocol($state, $sequence, $invocation, $double)]);
         }
 
@@ -975,12 +973,12 @@ private static function propertyState(object $double, string $property): array
         $state = $current->stateOf($double);
         $context = $current;
 
-        if ($state === null) {
+        if (!$state instanceof \Rasuvaeff\Understudy\Runtime\DoubleState) {
             $context = self::ownerOf($double) ?? $current;
             $state = $context->stateOf($double);
         }
 
-        if ($state === null) {
+        if (!$state instanceof \Rasuvaeff\Understudy\Runtime\DoubleState) {
             throw ForgottenDouble::propertyAfterReset($property);
         }
 
@@ -1014,7 +1012,7 @@ public static function referenceSlot(object $double, string $method, array $args
         // and the slot would keep the mode's own value instead of the
         // configured one. A recording phase is the exception — the sentinel
         // is what it came for.
-        if ($state !== null && !self::current()->isRecording()) {
+        if ($state instanceof \Rasuvaeff\Understudy\Runtime\DoubleState && !self::current()->isRecording()) {
             self::materializeOmittedArguments($state->blueprint->method($method), $method, $args);
         }
 
@@ -1036,7 +1034,7 @@ public static function referenceSlot(object $double, string $method, array $args
         $context = self::ownerOf($double) ?? self::current();
         $state = $context->stateOf($double);
 
-        if ($state === null) {
+        if (!$state instanceof \Rasuvaeff\Understudy\Runtime\DoubleState) {
             throw ForgottenDouble::afterReset($method);
         }
 
@@ -1064,7 +1062,7 @@ public static function callOriginal(object $double, string $method, array $args)
         $context = self::ownerOf($double) ?? self::current();
         $state = $context->stateOf($double);
 
-        if ($state === null) {
+        if (!$state instanceof \Rasuvaeff\Understudy\Runtime\DoubleState) {
             throw ForgottenDouble::afterReset($method);
         }
 
@@ -1139,7 +1137,7 @@ private static function runInline(DoubleState $state, object $double, string $me
             return $value;
         }
 
-        if ($value instanceof $double && self::ownerOf($value) === null) {
+        if ($value instanceof $double && !self::ownerOf($value) instanceof \Rasuvaeff\Understudy\Runtime\RuntimeContext) {
             throw OriginalReturnTypeViolation::foreignSelf($state->label(), $method);
         }
 
@@ -1446,7 +1444,7 @@ private static function materializeOmittedArguments(?MethodSignature $signature,
                 continue;
             }
 
-            if ($signature === null || !$signature->isOptional($position)) {
+            if (!$signature instanceof \Rasuvaeff\Understudy\Codegen\MethodSignature || !$signature->isOptional($position)) {
                 throw new \ArgumentCountError(sprintf(
                     'Too few arguments to function %s(), argument #%d not passed',
                     $method,
@@ -1484,7 +1482,7 @@ public static function reset(): void
     {
         $fiber = \Fiber::getCurrent();
 
-        if ($fiber === null) {
+        if (!$fiber instanceof \Fiber) {
             $stack = self::$main;
         } else {
             $fibers = self::fibers();
@@ -1509,7 +1507,7 @@ public static function reset(): void
         // plain array because the offset is a variable.
         /** @var list<RuntimeContext> $stack */
 
-        if ($fiber === null) {
+        if (!$fiber instanceof \Fiber) {
             self::$main = $stack;
 
             return;
@@ -1574,7 +1572,7 @@ private static function retire(RuntimeContext $context, bool $byScope = false):
             $captor->discard();
         }
 
-        if (self::$owners === null) {
+        if (!self::$owners instanceof \WeakMap) {
             return;
         }
 
diff --git a/src/Runtime/RuntimeContext.php b/src/Runtime/RuntimeContext.php
index 8b6ca55..178c790 100644
--- a/src/Runtime/RuntimeContext.php
+++ b/src/Runtime/RuntimeContext.php
@@ -191,7 +191,7 @@ public function allDoubles(): array
 
     public function rememberCaptor(Captor $captor): void
     {
-        if ($this->captors === null) {
+        if (!$this->captors instanceof \SplObjectStorage) {
             /** @var \SplObjectStorage<Captor, null> $captors */
             $captors = new \SplObjectStorage();
             $this->captors = $captors;
@@ -207,7 +207,7 @@ public function rememberCaptor(Captor $captor): void
      */
     public function captors(): array
     {
-        if ($this->captors === null) {
+        if (!$this->captors instanceof \SplObjectStorage) {
             return [];
         }
 
diff --git a/src/Understudy.php b/src/Understudy.php
index afb0d00..f923b82 100644
--- a/src/Understudy.php
+++ b/src/Understudy.php
@@ -169,9 +169,9 @@ private static function returnContract(DoubleState $state, string $method): ?Ret
     {
         $signature = $state->blueprint->method($method);
 
-        return $signature === null
-            ? null
-            : new ReturnContract($state->label(), $method, $signature, $state->blueprint->contracts[0]);
+        return $signature instanceof \Rasuvaeff\Understudy\Codegen\MethodSignature
+            ? new ReturnContract($state->label(), $method, $signature, $state->blueprint->contracts[0])
+            : null;
     }
 
     /**
@@ -260,7 +260,7 @@ private static function failuresIn(RuntimeContext $context, bool $strictStubs):
 
                 $failure = self::checkExpectation($state, $expectation, $strictStubs);
 
-                if ($failure !== null) {
+                if ($failure instanceof \Rasuvaeff\Understudy\VerificationFailure) {
                     $contextFailures[] = $failure;
                 }
             }
@@ -276,7 +276,7 @@ private static function failuresIn(RuntimeContext $context, bool $strictStubs):
 
         $outOfOrder = self::orderingFailure($ordered);
 
-        if ($outOfOrder !== null) {
+        if ($outOfOrder instanceof \Rasuvaeff\Understudy\VerificationFailure) {
             $failures[] = $outOfOrder;
         }
 
@@ -286,13 +286,13 @@ private static function failuresIn(RuntimeContext $context, bool $strictStubs):
         // would pass in silence. Whether one is armed is the check; a
         // complete protocol is a check that passed, an unfinished one the
         // failure below.
-        if ($context->armed !== null) {
+        if ($context->armed instanceof \Rasuvaeff\Understudy\Runtime\ArmedSequence) {
             $checks++;
         }
 
         $unfinished = self::checkArmedSequence($context);
 
-        if ($unfinished !== null) {
+        if ($unfinished instanceof \Rasuvaeff\Understudy\VerificationFailure) {
             $failures[] = $unfinished;
         }
 
@@ -315,7 +315,7 @@ private static function orderedExpectations(?DoubleState $only = null, ?RuntimeC
         $ordered = [];
 
         foreach (($context ?? Runtime::current())->allStates() as $state) {
-            if ($only !== null && $state !== $only) {
+            if ($only instanceof \Rasuvaeff\Understudy\Runtime\DoubleState && $state !== $only) {
                 continue;
             }
 
@@ -391,12 +391,12 @@ private static function checkArmedSequence(RuntimeContext $context): ?Verificati
     {
         $sequence = $context->armed;
 
-        if ($sequence === null || $sequence->isComplete()) {
+        if (!$sequence instanceof \Rasuvaeff\Understudy\Runtime\ArmedSequence || $sequence->isComplete()) {
             return null;
         }
 
         $pending = $sequence->pending();
-        \assert($pending !== null);
+        \assert($pending instanceof \Rasuvaeff\Understudy\Expectation\Expectation);
 
         return ArgumentFormatter::scope(static fn(): VerificationFailure => new VerificationFailure(
             kind: FailureKind::OutOfSequence,
@@ -421,7 +421,7 @@ private static function checkExpectation(DoubleState $state, Expectation $expect
         $cardinality = $expectation->cardinality();
         $count = $expectation->matchCount();
 
-        if ($cardinality === null) {
+        if (!$cardinality instanceof \Rasuvaeff\Understudy\Cardinality) {
             // A plain stub is permission, not a claim — unless strict stubs
             // turn "configured but never used" into a failure of its own.
             return $strictStubs && $count === 0
@@ -708,7 +708,7 @@ public static function forwarding(object $double, ?object $real = null): void
         if ($real !== null) {
             // A double delegating to a double — itself included — sends every
             // call back into the dispatcher it came from.
-            if (Runtime::ownerOf($real) !== null) {
+            if (Runtime::ownerOf($real) instanceof \Rasuvaeff\Understudy\Runtime\RuntimeContext) {
                 throw ForwardingTargetMismatch::understudyTarget($state->label());
             }
 
@@ -1076,14 +1076,14 @@ public static function allVerified(object $double): void
             foreach ($state->expectations() as $expectation) {
                 $failure = self::checkExpectation($state, $expectation, strictStubs: false);
 
-                if ($failure !== null) {
+                if ($failure instanceof \Rasuvaeff\Understudy\VerificationFailure) {
                     $failures[] = $failure;
                 }
             }
 
             $outOfOrder = self::orderingFailure(self::orderedExpectations($state));
 
-            if ($outOfOrder !== null) {
+            if ($outOfOrder instanceof \Rasuvaeff\Understudy\VerificationFailure) {
                 $failures[] = $outOfOrder;
             }
 
@@ -1159,7 +1159,7 @@ public static function expectSequence(callable ...$calls): void
         // Only a concurrent one is refused. A protocol that ran to completion
         // has nothing left to disagree about, and a two-phase test must be
         // able to describe its second phase.
-        if ($armed !== null && !$armed->isComplete()) {
+        if ($armed instanceof \Rasuvaeff\Understudy\Runtime\ArmedSequence && !$armed->isComplete()) {
             throw InvalidCallSpecification::protocolAlreadyArmed($armed->position(), $armed->length());
         }
 
@@ -1200,7 +1200,7 @@ public static function verifySequence(callable ...$calls): void
 
         $mismatch = self::firstSequenceMismatch($probes, $log);
 
-        if ($mismatch !== null) {
+        if ($mismatch instanceof \Rasuvaeff\Understudy\VerificationFailure) {
             throw VerificationFailed::of([$mismatch]);
         }
 
@@ -1548,7 +1548,7 @@ private static function stateOf(object $double, string $facade): DoubleState
     {
         $state = Runtime::stateOf($double);
 
-        if ($state === null) {
+        if (!$state instanceof \Rasuvaeff\Understudy\Runtime\DoubleState) {
             if (Runtime::isRetiredOnPurpose($double)) {
                 throw ForgottenDouble::retired();
             }
diff --git a/src/WhenBuilder.php b/src/WhenBuilder.php
index be4baea..8a2f9a3 100644
--- a/src/WhenBuilder.php
+++ b/src/WhenBuilder.php
@@ -71,7 +71,7 @@ public function returns(mixed ...$values): static
             throw InvalidSpecificationArgument::noReturnValues();
         }
 
-        if ($this->contract !== null) {
+        if ($this->contract instanceof \Rasuvaeff\Understudy\Expectation\ReturnContract) {
             // `returns(null)` on a void method is the idiom for "answer
             // nothing", and PHP itself reads a void method as answering null;
             // only a value that would be silently dropped is refused.
diff --git a/src/Wiring/Wire.php b/src/Wiring/Wire.php
index c60a019..a9dc94b 100644
--- a/src/Wiring/Wire.php
+++ b/src/Wiring/Wire.php
@@ -305,7 +305,7 @@ private static function matchesType(\ReflectionType $type, mixed $value): bool
 
     private static function typeName(?\ReflectionType $type): string
     {
-        return $type === null ? 'mixed' : (string) $type;
+        return $type instanceof \ReflectionType ? (string) $type : 'mixed';
     }
 
     /**
diff --git a/tests/Support/DispatchFindCommand.php b/tests/Support/DispatchFindCommand.php
index 57f9586..ea4887b 100644
--- a/tests/Support/DispatchFindCommand.php
+++ b/tests/Support/DispatchFindCommand.php
@@ -47,7 +47,7 @@ public function run(mixed $model, mixed $system): array
 
         $answering = $model->answerFor($this->id);
 
-        if ($answering === null) {
+        if (!$answering instanceof \Rasuvaeff\Understudy\Tests\Support\EngineSpec) {
             ++$system->unmatchedDefaults;
         } elseif ($answering->hasAction()) {
             ++$system->stubAnswers;
@@ -74,7 +74,7 @@ public function postCondition(mixed $model, mixed $result): bool
         }
 
         $answering = $model->answerFor($this->id);
-        $expectedTitle = $answering !== null && $answering->hasAction() ? $answering->answerTitle : null;
+        $expectedTitle = $answering instanceof \Rasuvaeff\Understudy\Tests\Support\EngineSpec && $answering->hasAction() ? $answering->answerTitle : null;
 
         return $title === $expectedTitle;
     }