Error in Asterisk 1.8 while dial to a unregistered extension

Greetings, I have a problem in Asterisk 1.8, when calling an unregistered user, I get this error "WARNING [6847]: chan_sip.c: 3351 __sip_xmit: sip_xmit of 0x7f3a0c010e70 (len 786) to return - 1: Invalid argument ", this did not happen in asterisk 1.6. I can do to fix this?

You can edit logger.conf to not print out WARNING level messages.

Or you can edit the source code to reduce this to an INFORMATION level one.