diff --git a/numeric.c b/numeric.c
index c207fbf521..3ceb5ae2d2 100644
--- a/numeric.c
+++ b/numeric.c
@@ -317,9 +317,9 @@ num_sadded(VALUE x, VALUE name)
     /* ruby_frame = ruby_frame->prev; */ /* pop frame for "singleton_method_added" */
     rb_remove_method_id(rb_singleton_class(x), mid);
     rb_raise(rb_eTypeError,
-	     "can't define singleton method \"%s\" for %s",
-	     rb_id2name(mid),
-	     rb_obj_classname(x));
+	     "can't define singleton method \"%"PRIsVALUE"\" for %"PRIsVALUE,
+	     rb_id2str(mid),
+	     rb_obj_class(x));
 
     UNREACHABLE;
 }
diff --git a/test/ruby/test_numeric.rb b/test/ruby/test_numeric.rb
index fc4f39a91d..7fbb8e955d 100644
--- a/test/ruby/test_numeric.rb
+++ b/test/ruby/test_numeric.rb
@@ -58,8 +58,8 @@ class TestNumeric < Test::Unit::TestCase
 
   def test_numeric
     a = Numeric.new
-    assert_raise(TypeError) { def a.foo; end }
-    assert_raise(TypeError) { eval("def a.\u3042; end") }
+    assert_raise_with_message(TypeError, /foo/) { def a.foo; end }
+    assert_raise_with_message(TypeError, /\u3042/) { eval("def a.\u3042; end") }
     assert_raise(TypeError) { a.dup }
   end