diff --git a/gc.c b/gc.c
index 6c3da3bc81..634ba0d41c 100644
--- a/gc.c
+++ b/gc.c
@@ -6257,7 +6257,7 @@ Init_GC(void)
 	rb_define_method(rb_cWeakMap, "[]", wmap_aref, 1);
 	rb_define_method(rb_cWeakMap, "include?", wmap_has_key, 1);
 	rb_define_method(rb_cWeakMap, "member?", wmap_has_key, 1);
-	rb_define_method(rb_cWeakMap, "key?", wmap_has_key, 0);
+	rb_define_method(rb_cWeakMap, "key?", wmap_has_key, 1);
 	rb_define_method(rb_cWeakMap, "inspect", wmap_inspect, 0);
 	rb_define_method(rb_cWeakMap, "each", wmap_each, 0);
 	rb_define_method(rb_cWeakMap, "each_pair", wmap_each, 0);
diff --git a/test/test_weakref.rb b/test/test_weakref.rb
index d0d40d470b..27a8499615 100644
--- a/test/test_weakref.rb
+++ b/test/test_weakref.rb
@@ -18,9 +18,11 @@ class TestWeakRef < Test::Unit::TestCase
   def test_recycled
     weak, str = make_weakref
     assert_nothing_raised(WeakRef::RefError) {weak.to_s}
+    assert_predicate(weak, :weakref_alive?)
     ObjectSpace.garbage_collect
     ObjectSpace.garbage_collect
     assert_raise(WeakRef::RefError) {weak.to_s}
+    assert_not_predicate(weak, :weakref_alive?)
   end
 
   def test_not_reference_different_object