diff --git a/ChangeLog b/ChangeLog
index 3c00b060d7..dabb7125e6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Feb 12 03:14:40 2012  Eric Hodel  <drbrain@segment7.net>
+
+	* vm_eval.c (check_funcall):  Call respond_to? with matching arity for
+	  legacy single-argument implementations.  [ruby-trunk - Bug #6000]
+
 Sat Feb 11 12:04:05 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* compile.c (defined_expr): guard the whole expression.
diff --git a/test/ruby/test_object.rb b/test/ruby/test_object.rb
index 08d85559c4..eca1be78d5 100644
--- a/test/ruby/test_object.rb
+++ b/test/ruby/test_object.rb
@@ -446,6 +446,20 @@ class TestObject < Test::Unit::TestCase
     assert_equal([[:respond_to?, :to_ary, true]], called, bug5158)
   end
 
+  def test_implicit_respond_to_arity_1
+    p = Object.new
+
+    called = []
+    p.singleton_class.class_eval do
+      define_method(:respond_to?) do |a|
+        called << [:respond_to?, a]
+        false
+      end
+    end
+    [[p]].flatten
+    assert_equal([[:respond_to?, :to_ary]], called, '[bug:6000]')
+  end
+
   def test_method_missing_passed_block
     bug5731 = '[ruby-dev:44961]'
 
diff --git a/vm_eval.c b/vm_eval.c
index a53f8f614c..aa326216b9 100644
--- a/vm_eval.c
+++ b/vm_eval.c
@@ -274,10 +274,13 @@ check_funcall(VALUE recv, ID mid, int argc, VALUE *argv)
     me = rb_method_entry(klass, idRespond_to);
     if (me && !(me->flag & NOEX_BASIC)) {
 	VALUE args[2];
+	int arity = rb_method_entry_arity(me);
+
+	if (arity < 1 || arity > 3) arity = 2;
 
 	args[0] = ID2SYM(mid);
 	args[1] = Qtrue;
-	if (!RTEST(vm_call0(th, recv, idRespond_to, 2, args, me))) {
+	if (!RTEST(vm_call0(th, recv, idRespond_to, arity, args, me))) {
 	    return Qundef;
 	}
     }