Disallow u"..." + b"..." and b"..." + u"...".
diff --git a/Lib/test/test_bytes.py b/Lib/test/test_bytes.py
index 1d826b6..999346f 100644
--- a/Lib/test/test_bytes.py
+++ b/Lib/test/test_bytes.py
@@ -374,7 +374,7 @@
         self.assertEqual(b1 + "def", bytes("abcdef"))
         self.assertEqual("def" + b1, bytes("defabc"))
         self.assertRaises(TypeError, lambda: b1 + u"def")
-        ##self.assertRaises(TypeError, lambda: u"abc" + b2)  # XXX FIXME
+        self.assertRaises(TypeError, lambda: u"abc" + b2)
 
     def test_repeat(self):
         b = bytes("abc")