[Serusers] 477 Message by SER???

I am using SER 0.9.6 and just saw a strange 477 message sent by SER. When an INVITE arrives to SER, the SER is not able to find default route to proxy the INVITE according to the RURI and SER reply a 477. The SIP RFC does not define such a message. Is this SER's own "protocol"?

This is the RURI in the INVITE to SER: INVITE sip:3205@192.170.12.11 SIP/2.0

While there is no IP route to 192.170.12.11 on the Linux that running SER, SER reply: SIP/2.0 477 Unfortunately error on sending to next hop occurred (477/TM)

Ya, the problem can be easily fixed by adding a route to that 192.170.12.x subnet (for peering).

But I realy want to know how does this 477 come from? Anyone?

Thanks,

Pat