NULL To Header

Anybody know why I would suddenly start getting these messages from siproxd?

Oct 13 15:32:10 cerberus siproxd: security.c:268 ERROR:security check failed: NULL To Header
Oct 13 15:32:10 cerberus siproxd: siproxd.c:348 ERROR:security_check_sip() failed… this is not good

Only thing I did to the * box was install/update some FreePBX modules. Worked fine before. Not blaming FreePBX … may just be a coincidence.

Box has Centos5, * 1.4 and FreePBX 2.3 and sits behind Smoothwall Express 3.0 with sip proxy enabled and set for transparent mode.

The messages above are from the Smoothwall box /var/log/messages and result from a registration request from my * box.

Any pointers would be appreciated.