--- a/tests/test-encoding-func.py	Sun Apr 23 13:06:23 2017 +0900
+++ b/tests/test-encoding-func.py	Sun Apr 23 13:08:58 2017 +0900
@@ -34,6 +34,12 @@
         self.assertTrue(s is encoding.tolocal(s))
         self.assertTrue(s is encoding.fromlocal(s))
 
+class Utf8bEncodingTest(unittest.TestCase):
+    def testasciifastpath(self):
+        s = b'\0' * 100
+        self.assertTrue(s is encoding.toutf8b(s))
+        self.assertTrue(s is encoding.fromutf8b(s))
+
 if __name__ == '__main__':
     import silenttestrunner
     silenttestrunner.main(__name__)