When I ran an update, I get the following error.

```
GPG error: http://cran.wustl.edu maverick/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9

```

Therefore, I ran the following code and get the following error message:

```
$ gpg --keyserver subkeys.pgp.net --recv 51716619E084DAB9
gpg: requesting key E084DAB9 from hkp server subkeys.pgp.net
gpg: key E084DAB9: "Michael Rutter <marutter@gmail.com>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

```

How do I set a public key that works or what can I do to fix this problem?