Why Postfix always complain "unexpected EOF" with my own tcp_table program?

[Zhang Huangbin]

Dear all,

I wrote a simple daemon service in Python, it's used in Postfix transport_maps 
like this:

transport_maps = tcp:127.0.0.1:1234

It always returns '200 my_transport\n' as described in Postfix manual page 
tcp_table(5), but Postfix always complains "unexpected EOF" like below:

Jul 27 22:51:53 d7 postfix/trivial-rewrite[4260]: warning: read TCP map reply 
from 127.0.0.1:1234: unexpected EOF (Success)

This Python daemon server uses 'asynchat' module, and return '200 
my_transport\n' with 'async_chat.push()' method like this:

self.push('200 my_transport\n')

Any idea why Postfix always complain "unexpected EOF"?

Thanks for your time.