diff --git a/lib/net/imap/sequence_set.rb b/lib/net/imap/sequence_set.rb
index 453e51e91..483ed8031 100644
--- a/lib/net/imap/sequence_set.rb
+++ b/lib/net/imap/sequence_set.rb
@@ -2420,7 +2420,7 @@ def reverse_each_minmax_with_index(minmaxes)
       def slice_length(start, length)
         start  = Integer(start.to_int)
         length = Integer(length.to_int)
-        raise ArgumentError, "length must be positive" unless length.positive?
+        raise ArgumentError, "length must not be negative" if length.negative?
         stop   = start + length unless start.negative? && start.abs <= length
         slice_range(start...stop)
       end
diff --git a/test/net/imap/test_sequence_set.rb b/test/net/imap/test_sequence_set.rb
index b7955c00c..578534016 100644
--- a/test/net/imap/test_sequence_set.rb
+++ b/test/net/imap/test_sequence_set.rb
@@ -492,16 +492,10 @@ def pend_slice_bug(what, &) = pend("#slice bug: #{what}", &)
   def pend_slice_neg_len(&)
     pend_slice_bug("allow negative length", &)
     assert_raise_with_message(ArgumentError,
-                              "length must be positive",
+                              "length must not be negative",
                               &)
   end
 
-  def pend_slice_zero_len(&)
-    pend_slice_bug("allow zero length", &)
-    assert_raise_with_message(ArgumentError,
-                              "length must be positive",
-                              &)
-  end
 
   test "#slice(start, length) -> nil, for negative length" do
     set = SequenceSet[1...200]
@@ -511,22 +505,22 @@ def pend_slice_zero_len(&)
 
   test "#slice(start, length) -> nil, for invalid start" do
     set = SequenceSet[1..100]
-    pend_slice_zero_len do assert_nil set[ 101, 0] end
+    assert_nil set[ 101, 0]
     assert_nil set[ 101, 1]
     assert_nil set[ 101, 9]
-    pend_slice_zero_len do assert_nil set[-101, 0] end
+    assert_nil set[-101, 0]
     assert_nil set[-101, 1]
     assert_nil set[-101, 9]
 
     set = SequenceSet[*((10..100) % 10)]
     assert_nil set[-11, 1]
-    pend_slice_zero_len do assert_nil set[-11, 0] end
+    assert_nil set[-11, 0]
   end
 
   test "#slice(start, length) -> empty, for valid start but zero length" do
     set = SequenceSet[1...200]
-    pend_slice_zero_len do assert_same SequenceSet.empty, set[ 100, 0] end
-    pend_slice_zero_len do assert_same SequenceSet.empty, set[-100, 0] end
+    assert_same SequenceSet.empty, set[ 100, 0]
+    assert_same SequenceSet.empty, set[-100, 0]
   end
 
   test "#[range] -> set, for valid range" do
@@ -613,7 +607,7 @@ def pend_slice_zero_len(&)
     assert_equal SequenceSet[    100], set[ 9, 4]
     assert_equal SequenceSet[    100], set[-1, 4]
     # when positive start == cardinality
-    pend_slice_zero_len do assert_equal SequenceSet.empty,    set[10, 0] end
+    assert_equal SequenceSet.empty,    set[10, 0]
     assert_equal SequenceSet.empty,    set[10, 4]
     assert_equal SequenceSet.empty,    set[10...10]
     assert_equal SequenceSet.empty,    set[10.. 10]