[Users] BYE then Forbidden

[unplug]

Why a forbidden will be issued from openser after it receiving a BYE message?

UA				               openser
|>F21 BYE --------------------------------------->|
|<----------------------------- Forbidden 403 F22<|

     SIP MESSAGE 21       210.184.23.31:5060() -> 203.193.26.234:5060()
     UDP Frame 22       18/May/06 17:15:33.7124
TimeFromPreviousSipFrame=3.2197 TimeFromStart=18.3328
BYE sip:936418567 at 203.193.26.242:5060 SIP/2.0
Via: SIP/2.0/UDP 10.0.0.27:5060;branch=z9hG4bK-1-10
From: sipp <sip:871963472069 at 10.0.0.27:5060>;tag=1
To: sut <sip:36418587 at 203.193.26.234:5060>;tag=SIPaPOFVaXMKm7Q3
Call-ID: 1-2419 at 10.0.0.27
CSeq: 2 BYE
Contact: sip:871963472069 at 10.0.0.27:5060
Max-Forwards: 70
Subject: Performance Test
Content-Length: 0


     SIP MESSAGE 22       203.193.26.234:5060() -> 210.184.23.31:5060()
     UDP Frame 23       18/May/06 17:15:33.7181
TimeFromPreviousSipFrame=0.0058 TimeFromStart=18.3386
SIP/2.0 403 Forbidden
Via: SIP/2.0/UDP
10.0.0.27:5060;branch=z9hG4bK-1-10;rport=5060;received=210.184.23.31
From: sipp <sip:871963472069 at 10.0.0.27:5060>;tag=1
To: sut <sip:36418587 at 203.193.26.234:5060>;tag=SIPaPOFVaXMKm7Q3
Call-ID: 1-2419 at 10.0.0.27
CSeq: 2 BYE
Server: OpenSer (1.0.1 (i386/linux))
Content-Length: 0
Warning: 392 203.193.26.234:5060 "Noisy feedback tells:  pid=23798
req_src_ip=210.184.23.31 req_src_port=5060
in_uri=sip:936418587 at 203.193.26.242:5060
out_uri=sip:936418587 at 203.193.26.242:5060 via_cnt==1"