diff --git a/paramiko/pkey.py b/paramiko/pkey.py
index e925bb6..cbc1837 100644
--- a/paramiko/pkey.py
+++ b/paramiko/pkey.py
@@ -26,6 +26,7 @@ import os
 
 from Crypto.Hash import MD5
 from Crypto.Cipher import DES3, AES
+import six
 
 from paramiko.common import *
 from paramiko import util
@@ -303,7 +304,7 @@ class PKey (object):
             end += 1
         # if we trudged to the end of the file, just try to cope.
         try:
-            data = base64.decodestring(''.join(lines[start:end]))
+            data = base64.decodestring(six.b(''.join(lines[start:end])))
         except base64.binascii.Error as e:
             raise SSHException('base64 decoding error: ' + str(e))
         if 'proc-type' not in headers: