diff --git a/activesupport/test/core_ext/duration_test.rb b/activesupport/test/core_ext/duration_test.rb
index 26a9ea4aeeb1ae2ca36eec0e430ea20188c0f1b3..6f7f16da5bb0fb40bc39998ed868fbeccecd50e8 100644
--- a/activesupport/test/core_ext/duration_test.rb
+++ b/activesupport/test/core_ext/duration_test.rb
@@ -21,7 +21,7 @@ class << k; undef_method :== end
   end
 
   def test_instance_of
-    assert 1.minute.instance_of?(Fixnum)
+    assert 1.minute.instance_of?(1.class)
     assert 2.days.instance_of?(ActiveSupport::Duration)
     assert !3.second.instance_of?(Numeric)
   end