OK response to MESSAGE with no tag in To header

[rui.macedo@xxxxxxxxxxxxxxxx (Rui Macedo)]

Hi,

I'm having a problem with the OK response, in my SIP client.
When I receive a MESSAGE, my UAS responds with a 200 OK but the header 
To does not contain a tag (defined on RFC 3261 section 8.2.6.2).

This results in a problem, when the OK reaches the server it is ignored 
because it doesn't comply with the RCF standard.

Any ideas of what can cause this issue?

Thanks,
Rui Macedo