Passthrough mode - I receive ERROR when I send a packet

[vapisoft]

I connect to TCP with AT+CIPSTART
And then I enter oassthrough mode with: AT+CIPMODE=1
I receive packets ok.
But
WhenI send a packet I get an error: ERROR (as if I am not in passthrough mode).
I tried sending: AT+SIPSEND (without = length) before the AT+CIPMODE and after it, and I still get the ERROR response.
How do I send a packet from the client to the server in passthrough mode?