"sip unregister" gets unexpected "Peer unknown"

Does anyone see what I’m doing wrong here with the "sip unregister command?
*CLI> sip unregister 51210
Peer unknown: ‘51210’. Not unregistered.

*CLI> sip show registry
Host dnsmgr Username Refresh State Reg.Time
152.148.200.236:5060 N 51210 3585 Registered Mon, 08 Dec 2014 10:30:01
152.148.200.236:5060 N 51209 3585 Registered Mon, 08 Dec 2014 10:30:01
152.148.200.236:5060 N 51208 3585 Registered Mon, 08 Dec 2014 10:30:01
152.148.200.236:5060 N 51207 3585 Registered Mon, 08 Dec 2014 10:30:01
152.148.200.236:5060 N 51206 3585 Registered Mon, 08 Dec 2014 10:30:01
192.168.90.50:5260 N 62025 1785 Registered Mon, 08 Dec 2014 10:30:01
192.168.90.50:5260 N 62024 1785 Registered Mon, 08 Dec 2014 10:30:01
192.168.90.50:5260 N 62023 1785 Registered Mon, 08 Dec 2014 10:30:01
192.168.90.50:5260 N 62022 1785 Registered Mon, 08 Dec 2014 10:30:02
192.168.90.50:5260 N 62021 1785 Registered Mon, 08 Dec 2014 10:30:02
10 SIP registrations.