error: bad char in struct format

We're occasionally seeing this error in production.

error: bad char in struct format
...snip...
File "bmemcached/client.py", line 104, in get
value, cas = self.get_server(key).get(key)
File "bmemcached/protocol.py", line 379, in get
flags, value = struct.unpack('!L%ds' % (bodylen - 4, ), extra_content)

It might be some missing data in buffer that we did not read. Or it could be a race condition.
Can you save the payload or do something to reproduce this?
Thank you