Index: Lib/smtplib.py
===================================================================
--- Lib/smtplib.py	(revision 71888)
+++ Lib/smtplib.py	(working copy)
@@ -545,7 +545,8 @@
             return encode_base64(response)
 
         def encode_plain(user, password):
-            return encode_base64("\0%s\0%s" % (user, password))
+            s = "\0%s\0%s" % (user, password)
+            return encode_base64(s.encode('ascii'), eol='')
 
 
         AUTH_PLAIN = "PLAIN"