Ubuntu 10.10 release-upgrade fails

Just tried to upgrade my ubuntu 10.10 using "do-release-upgrade" from console and fails:
Error is:

alex@spse:/# do-release-upgrade
Checking for a new ubuntu release
Get:1 Upgrade tool signature [198B]
Get:2 Upgrade tool [1306kB]
Fetched 1307kB in 0s (0B/s)
authenticate 'natty.tar.gz' against 'natty.tar.gz.gpg'
exception from gpg: GnuPG exited non-zero, with code 2
Debug information:

gpg: Signature made Mon Jun  4 09:40:31 2012 EDT using DSA key ID 437D05B5
gpg: Can't check signature: public key not found
Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.
alex@spse:/#

Any ideas?

Thanks,

Alex

Using Eliah Kagan link then command:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 437D05B5 3E5C1192

do-release-upgrade seems to work;

Even so, I don't know how safe it is to upgrade over ssh.
My current config is: ubuntu 10.10 installed on physical disk, and booted with vmware-player from windows 7.

Any ideas/suggestions?

Thanks,
Alex