diff --git a/activesupport/CHANGELOG b/activesupport/CHANGELOG
index 6d3fd44368..e6e8f2d1ea 100644
--- a/activesupport/CHANGELOG
+++ b/activesupport/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Fix Object.subclasses_of to only return currently defined objects [Jonathan Viney <jonathan@bluewire.net.nz>]
+
 * Fix constantize to properly handle names beginning with '::'. [Nicholas Seckar]
 
 * Make String#last return the string instead of nil when it is shorter than the limit [Scott Barron].
diff --git a/activesupport/lib/active_support/core_ext/object/extending.rb b/activesupport/lib/active_support/core_ext/object/extending.rb
index 8c638f83c2..9abdfc0bc9 100644
--- a/activesupport/lib/active_support/core_ext/object/extending.rb
+++ b/activesupport/lib/active_support/core_ext/object/extending.rb
@@ -8,7 +8,7 @@ class Object #:nodoc:
   def subclasses_of(*superclasses)
     subclasses = []
     ObjectSpace.each_object(Class) do |k|
-      next if (k.ancestors & superclasses).empty? || superclasses.include?(k) || k.to_s.include?("::") || subclasses.include?(k)
+      next if (k.ancestors & superclasses).empty? || superclasses.include?(k) || k.to_s.include?("::") || subclasses.include?(k) || !Object.const_defined?(k.to_s.to_sym)
       subclasses << k
     end
     subclasses
@@ -42,4 +42,4 @@ class Object #:nodoc:
       block.bind(self)[*arguments]
     end
   end
-end
\ No newline at end of file
+end
diff --git a/activesupport/test/core_ext/object_and_class_ext_test.rb b/activesupport/test/core_ext/object_and_class_ext_test.rb
index 5d7a06d08d..6a100e813b 100644
--- a/activesupport/test/core_ext/object_and_class_ext_test.rb
+++ b/activesupport/test/core_ext/object_and_class_ext_test.rb
@@ -7,6 +7,9 @@ class ClassB < ClassA; end
 class ClassC < ClassB; end
 class ClassD < ClassA; end
 
+class ClassI; end
+class ClassJ < ClassI; end
+
 module Bar
   def bar; end
 end
@@ -31,6 +34,12 @@ class ClassExtTest < Test::Unit::TestCase
     assert !defined?(ClassC)
     assert !defined?(ClassD)
   end
+  
+  def test_subclasses_of
+    assert_equal [ClassJ], Object.subclasses_of(ClassI)
+    ClassI.remove_subclasses
+    assert_equal [], Object.subclasses_of(ClassI)
+  end
 end
 
 class ObjectTests < Test::Unit::TestCase