diff --git a/cj-btc-json/build.gradle b/cj-btc-json/build.gradle
index ac89bb0bf..c7d611148 100644
--- a/cj-btc-json/build.gradle
+++ b/cj-btc-json/build.gradle
@@ -1,13 +1,26 @@
+import net.ltgt.gradle.errorprone.CheckSeverity
+
 plugins {
     id 'java-library'
 }
+apply plugin: 'net.ltgt.errorprone'
 
 ext.moduleName = 'org.consensusj.bitcoin.json'
 
 dependencies {
+    api libs.jspecify
     api libs.bitcoinj.core
     api libs.jackson.core
     api libs.jackson.databind
+
+    errorprone libs.errorprone.core
+}
+
+tasks.withType(JavaCompile) {
+    options.errorprone {
+        check("NullAway", CheckSeverity.ERROR)
+        option("NullAway:OnlyNullMarked")
+    }
 }
 
 jar {
diff --git a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/conversion/RpcClientModule.java b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/conversion/RpcClientModule.java
index 24cd9d485..69aa1bbf8 100644
--- a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/conversion/RpcClientModule.java
+++ b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/conversion/RpcClientModule.java
@@ -24,6 +24,7 @@
 import org.bitcoinj.crypto.ECKey;
 import org.bitcoinj.base.Sha256Hash;
 import org.bitcoinj.core.Transaction;
+import org.jspecify.annotations.Nullable;
 
 import java.util.Objects;
 
@@ -43,7 +44,7 @@ public RpcClientModule() {
      * @param network Which network we are going to be a client for (may be null if {@code strictAddressParsing} is false)
      * @param strictAddressParsing set to {@code true} to throw exceptions when deserializing addresses that don't match {@code network}
      */
-    protected RpcClientModule(Network network, boolean strictAddressParsing) {
+    protected RpcClientModule(@Nullable Network network, boolean strictAddressParsing) {
         super("BitcoinJMappingClient", new Version(1, 0, 0, null, null, null));
 
         this.addDeserializer(Address.class, strictAddressParsing ? new AddressDeserializer(Objects.requireNonNull(network)) : new AddressDeserializer())
diff --git a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/conversion/package-info.java b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/conversion/package-info.java
index 838075b7a..9ab390321 100644
--- a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/conversion/package-info.java
+++ b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/conversion/package-info.java
@@ -16,4 +16,7 @@
 /**
  * Jackson JSON serializers, deserializers, modules, and utilities.
  */
-package org.consensusj.bitcoin.json.conversion;
\ No newline at end of file
+@NullMarked
+package org.consensusj.bitcoin.json.conversion;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/AddressGroupingItem.java b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/AddressGroupingItem.java
index e893a91ba..335104d12 100644
--- a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/AddressGroupingItem.java
+++ b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/AddressGroupingItem.java
@@ -19,6 +19,7 @@
 import org.bitcoinj.base.Address;
 import org.bitcoinj.base.AddressParser;
 import org.bitcoinj.base.Coin;
+import org.jspecify.annotations.Nullable;
 
 import java.util.List;
 
@@ -30,7 +31,7 @@ public class AddressGroupingItem {
     private static final AddressParser addressParser = AddressParser.getDefault();
     private final Address address;
     private final Coin balance;
-    private final String account;
+    @Nullable private final String account;
 
     public AddressGroupingItem(Address address, Coin balance, String account) {
         this.address = address;
@@ -57,6 +58,7 @@ public Coin getBalance() {
         return balance;
     }
 
+    @Nullable
     public String getAccount() {
         return account;
     }
diff --git a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/AddressInfo.java b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/AddressInfo.java
index 4e73b68a5..67e3f6820 100644
--- a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/AddressInfo.java
+++ b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/AddressInfo.java
@@ -21,6 +21,7 @@
 
 import com.fasterxml.jackson.annotation.JsonProperty;
 import org.bitcoinj.base.Address;
+import org.jspecify.annotations.Nullable;
 
 public class AddressInfo {
     private final Address address;
@@ -35,6 +36,7 @@ public class AddressInfo {
     private final boolean iscompressed;
     private final String label;
     private final boolean ischange;
+    @Nullable
     private final Instant timestamp;
     private final List<Object> labels;
 
diff --git a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/RawTransactionInfo.java b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/RawTransactionInfo.java
index e6501afb3..09017f1bc 100644
--- a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/RawTransactionInfo.java
+++ b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/RawTransactionInfo.java
@@ -24,6 +24,7 @@
 import org.bitcoinj.base.Coin;
 import org.bitcoinj.base.Sha256Hash;
 import org.bitcoinj.core.Transaction;
+import org.jspecify.annotations.Nullable;
 
 import java.time.Instant;
 import java.util.ArrayList;
@@ -33,7 +34,6 @@
 /**
  * RawTransaction POJO
  */
-// "hash" property added (present in Bitcoin 0.13)
 public class RawTransactionInfo {
     private final String hex;
     private final Sha256Hash txid;
@@ -41,6 +41,7 @@ public class RawTransactionInfo {
     private final LockTime lockTime;
     private final List<Vin> vin;
     private final List<Vout> vout;
+    @Nullable
     private final Sha256Hash blockhash;
     private final int confirmations;
     private final Instant time;
@@ -130,6 +131,7 @@ public List<Vout> getVout() {
         return vout;
     }
 
+    @Nullable
     public Sha256Hash getBlockhash() {
         return blockhash;
     }
diff --git a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/bitcore/package-info.java b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/bitcore/package-info.java
index 66801a942..a4104f3a6 100644
--- a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/bitcore/package-info.java
+++ b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/bitcore/package-info.java
@@ -21,4 +21,4 @@
  * <p>
  * <a href="https://github.com/OmniLayer/omnicore/pull/1180">OmniCore PR #1180</a>
  */
-package org.consensusj.bitcoin.json.pojo.bitcore;
\ No newline at end of file
+package org.consensusj.bitcoin.json.pojo.bitcore;
diff --git a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/package-info.java b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/package-info.java
index 4b2f33f0f..134e57f2f 100644
--- a/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/package-info.java
+++ b/cj-btc-json/src/main/java/org/consensusj/bitcoin/json/pojo/package-info.java
@@ -16,4 +16,4 @@
 /**
  * POJOs definitions for JSON objects
  */
-package org.consensusj.bitcoin.json.pojo;
\ No newline at end of file
+package org.consensusj.bitcoin.json.pojo;
diff --git a/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/DefaultRpcClient.java b/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/DefaultRpcClient.java
index 993bd72ed..b578a61df 100644
--- a/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/DefaultRpcClient.java
+++ b/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/DefaultRpcClient.java
@@ -19,6 +19,7 @@
 import com.fasterxml.jackson.databind.JavaType;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import org.jspecify.annotations.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -86,14 +87,14 @@ public interface TransportFactory {
     private final JavaType defaultType;
     private final JsonRpcTransport<JavaType> transport;
 
-    public DefaultRpcClient(URI server, final String rpcUser, final String rpcPassword) {
+    public DefaultRpcClient(URI server, @Nullable String rpcUser, @Nullable String rpcPassword) {
         this(JsonRpcTransport.getDefaultSSLContext(), DEFAULT_JSON_RPC_VERSION, server, rpcUser, rpcPassword);
     }
 
-    public DefaultRpcClient(JsonRpcMessage.Version jsonRpcVersion, URI server, final String rpcUser, final String rpcPassword) {
+    public DefaultRpcClient(JsonRpcMessage.Version jsonRpcVersion, URI server, @Nullable String rpcUser, @Nullable String rpcPassword) {
         this(JsonRpcTransport.getDefaultSSLContext(), jsonRpcVersion, server, rpcUser, rpcPassword);
     }
-    public DefaultRpcClient(SSLContext sslContext, JsonRpcMessage.Version jsonRpcVersion, URI server, final String rpcUser, final String rpcPassword) {
+    public DefaultRpcClient(SSLContext sslContext, JsonRpcMessage.Version jsonRpcVersion, URI server, @Nullable String rpcUser, @Nullable String rpcPassword) {
         this((m) -> new JsonRpcClientJavaNet(m, sslContext, server, rpcUser, rpcPassword), jsonRpcVersion);
     }
 
diff --git a/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/JsonRpcClient.java b/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/JsonRpcClient.java
index 46cc2eacb..9de11ff17 100644
--- a/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/JsonRpcClient.java
+++ b/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/JsonRpcClient.java
@@ -15,6 +15,8 @@
  */
 package org.consensusj.jsonrpc;
 
+import org.jspecify.annotations.Nullable;
+
 import java.io.IOException;
 import java.lang.reflect.Type;
 import java.util.Arrays;
@@ -84,15 +86,15 @@ default <R> CompletableFuture<R>  sendAsync(String method, List<Object> params)
      * @throws IOException network error
      * @throws JsonRpcStatusException JSON RPC status error
      */
-    default Object send(String method, Object... params) throws IOException, JsonRpcStatusException {
+    default Object send(String method, @Nullable Object... params) throws IOException, JsonRpcStatusException {
         return send(method, Arrays.asList(params));
     }
 
-    default <R> R send(String method, Class<R> resultType, Object... params) throws IOException, JsonRpcStatusException {
+    default <R> R send(String method, Class<R> resultType, @Nullable Object... params) throws IOException, JsonRpcStatusException {
         return send(method, resultType, Arrays.asList(params));
     }
 
-    default <R> CompletableFuture<R> sendAsync(String method, Class<R> resultType, Object... params) {
+    default <R> CompletableFuture<R> sendAsync(String method, Class<R> resultType, @Nullable Object... params) {
         return sendAsync(method, resultType, Arrays.asList(params));
     }
 
@@ -132,11 +134,11 @@ default Object send(String method, T resultType, List<Object> params) throws IOE
     /**
      * Varargs version
      */
-    default Object send(String method, T resultType, Object... params) throws IOException, JsonRpcStatusException {
+    default Object send(String method, T resultType, @Nullable Object... params) throws IOException, JsonRpcStatusException {
         return syncGet(sendRequestForResultAsync(buildJsonRequest(method, params), resultType));
     }
 
-    default <R> CompletableFuture<R> sendAsync(String method, T resultType, Object... params) {
+    default <R> CompletableFuture<R> sendAsync(String method, T resultType, @Nullable Object... params) {
         return sendRequestForResultAsync(buildJsonRequest(method, params), resultType);
     }
 
@@ -167,7 +169,7 @@ default JsonRpcRequest buildJsonRequest(String method, List<Object> params) {
         return new JsonRpcRequest(getJsonRpcVersion(), method, params);
     }
 
-    default JsonRpcRequest buildJsonRequest(String method, Object... params) {
+    default JsonRpcRequest buildJsonRequest(String method, @Nullable Object... params) {
         return new JsonRpcRequest(getJsonRpcVersion(), method, Arrays.asList(params));
     }
 
diff --git a/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/JsonRpcClientJavaNet.java b/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/JsonRpcClientJavaNet.java
index 444136049..bfe27d2c3 100644
--- a/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/JsonRpcClientJavaNet.java
+++ b/consensusj-jsonrpc/src/main/java/org/consensusj/jsonrpc/JsonRpcClientJavaNet.java
@@ -18,6 +18,7 @@
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.JavaType;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import org.jspecify.annotations.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -42,7 +43,9 @@ public class JsonRpcClientJavaNet implements JsonRpcTransport<JavaType> {
 
     private final ObjectMapper mapper;
     private final URI serverURI;
+    @Nullable
     private final String username;
+    @Nullable
     private final String password;
     private final HttpClient client;
     private static final String UTF8 = StandardCharsets.UTF_8.name();
@@ -52,7 +55,7 @@ public JsonRpcClientJavaNet(ObjectMapper mapper, URI server, final String rpcUse
         this(mapper, JsonRpcTransport.getDefaultSSLContext(), server, rpcUser, rpcPassword);
     }
 
-    public JsonRpcClientJavaNet(ObjectMapper mapper, SSLContext sslContext, URI server, final String rpcUser, final String rpcPassword) {
+    public JsonRpcClientJavaNet(ObjectMapper mapper, SSLContext sslContext, URI server, @Nullable String rpcUser, @Nullable String rpcPassword) {
         log.debug("Constructing JSON-RPC client for: {}", server);
         this.mapper = mapper;
         this.serverURI = server;