gpg error - testing ?

[Mehmet Fatih AKBULUT]

hi all,
i am getting this when apt-get update :

W: GPG error: http://secure-testing.debian.net testing/security-updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 946AA6E18722E71E
W: You may want to run apt-get update to correct these problems

how can i fix this error ?