SSL Forwarder (for mitm testing)

I have the following Python code:

from OpenSSL import SSL
import thread, socket

TARGET_HOST = 'server.example.com'
TARGET_PORT = 443
CERT_FILE = 'server.pem'
LISTEN_PORT = 443

def forwarder(src, dst):
        data = ' '
        while data:
                data = src.recv(4096)
                print data
                if data:
                        dst.sendall(data)
                else:
                        src.shutdown(socket.SHUT_RD)
                        dst.shutdown(socket.SHUT_WR)

def sslforwarder((cli, endpoint)):
        print "connection from ", endpoint
        ctx = SSL.Context(SSL.SSLv23_METHOD)
        srv = SSL.Connection(ctx, socket.socket())
        srv.setblocking(1)
        srv.connect((TARGET_HOST, TARGET_PORT))
        thread.start_new_thread(forwarder, (cli, srv))
        thread.start_new_thread(forwarder, (srv, cli))

ctx = SSL.Context(SSL.SSLv23_METHOD)
ctx.use_privatekey_file(CERT_FILE)
ctx.use_certificate_file(CERT_FILE)
ss = SSL.Connection(ctx, socket.socket())
ss.setblocking(1)
ss.bind(('0.0.0.0', LISTEN_PORT))
ss.listen(5)

try:
        while True:
                sslforwarder(ss.accept())
except KeyboardInterrupt:
        raise SystemExit

but when I run it I get the error:

Fatal Python error: PyEval_RestoreThread: NULL tstate
Aborted

This occurs when the server is sending data back to the client. Any ideas?

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