diff -r ae047ebe10f9 Lib/email/encoders.py
--- a/Lib/email/encoders.py	Wed Dec 11 17:21:13 2013 -0600
+++ b/Lib/email/encoders.py	Thu Dec 12 14:15:54 2013 +0800
@@ -54,13 +54,10 @@
         # There's no payload.  For backwards compatibility we use 7bit
         msg['Content-Transfer-Encoding'] = '7bit'
         return
-    # We play a trick to make this go fast.  If encoding/decode to ASCII
+    # We play a trick to make this go fast.  If decoding to ASCII
     # succeeds, we know the data must be 7bit, otherwise treat it as 8bit.
     try:
-        if isinstance(orig, str):
-            orig.encode('ascii')
-        else:
-            orig.decode('ascii')
+        orig.decode('ascii')
     except UnicodeError:
         charset = msg.get_charset()
         output_cset = charset and charset.output_charset