Index: vm_eval.c
===================================================================
--- vm_eval.c	(revision 34549)
+++ vm_eval.c	(working copy)
@@ -274,10 +274,13 @@ check_funcall(VALUE recv, ID mid, int ar
     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;
 	}
     }
Index: test/ruby/test_object.rb
===================================================================
--- test/ruby/test_object.rb	(revision 34549)
+++ test/ruby/test_object.rb	(working copy)
@@ -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]'