python base64 decode incorrect padding错误解决方法

python的base64.decodestring方法做base64解码时报错：

代码如下:

Traceback (most recent call last):
File "/export/www/outofmemory.cn/controllers/user.py", line 136, in decryptPassword
encryptPwd = base64.b64decode(encryptPwd)
File "/usr/lib/python2.7/base64.py", line 76, in b64decode
raise TypeError(msg)
TypeError: Incorrect padding

代码如下:

def decode_base64(data):
"""Decode base64, padding being optional.

:param data: Base64 data as an ASCII byte string
:returns: The decoded byte string.

"""
missing_padding = 4 - len(data) % 4
if missing_padding:
data += b'='* missing_padding
return base64.decodestring(data)