[Serusers] Why does added headers show up in the CANCEL?

Hello! I'm playing around with my own module which is adding or replacing a header in proxied INVITE requests. I'm using the "parse_headers ... anchor_lump ... insert_new_lump_before" method on the proxied request and the outgoing INVITE looks great with the new header. But the request is forwarded by using the tm module and when the transaction receives a CANCEL the outgoing CANCEL gets the header added to the initial INVITE. That would not be a problem for a normal SIP UA, since unknown headers should be ignored, but this UA from a big telecom vendor throws a 400 Bad request. Why is the added headers added to the CANCEL? Shouldn't just the required and transaction identifying headers be enough in a CANCEL? Could I do anything different to prevent this from happening? /Henrik Gustafsson