raise a SyntaxError in detect_encoding() when a codec lookup fails like the builtin parser #4021
diff --git a/Lib/test/test_tokenize.py b/Lib/test/test_tokenize.py
index 8fbd216..75a7a80 100644
--- a/Lib/test/test_tokenize.py
+++ b/Lib/test/test_tokenize.py
@@ -795,6 +795,8 @@
         self.assertEquals(encoding, 'utf-8')
         self.assertEquals(consumed_lines, [])
 
+        readline = self.get_readline((b'# coding: bad\n',))
+        self.assertRaises(SyntaxError, detect_encoding, readline)
 
 class TestTokenize(TestCase):