I am trying to SSH from a NAS to a webserver using a public key. NAS user is 'root' and webserver user is 'backup'

I have all permissions set correctly and when I debug the SSH connection I get: (last little bit of the debug)

debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Next authentication method: publickey
debug1: Offering DSA public key: /root/.ssh/id_dsa.pub
debug1: Server accepts key: pkalg ssh-dss blen 433
debug1: key_parse_private_pem: PEM_read_PrivateKey failed
debug1: read PEM private key done: type <unknown>
Enter passphrase for key '/root/.ssh/id_dsa.pub':

I am using the command:

ssh -v -i /root/.ssh/id_dsa.pub [email protected]

The fact that it is asking for a passphrase is a good sign surely, but I do not want it to prompt for this or a password (which comes afterwards if I press 'return' on the passphrase)