asterisk users - Nov 2016 - Empty user string on pjsip inbound trunk

Hi,

I try to setup an inbound trunk using pjsip_wizard.conf. Now, when I 
receive a call from that trunk with an empty user string, and I try to 
match it with 's' in the dial plan, Asterisk reports that the extension 
was not found in the context.

* pjsip_wizard.conf:

[example]
type = wizard
sends_auth = no
sends_registrations = no
remote_hosts = example.com:5060
endpoint/context = from-extern

* extensions.conf:

[from-extern]
exten => s,1,Playback(demo-thanks)

* Asterisk log:

res_pjsip_session.c: Call from 'example' (UDP:111.222.3.4:5060) to 
extension '' rejected because extension not found in context
'from-extern'.


What am I doing wrong?


Regards,

Simon