When running sudo apt-get update, I get the following:

W: GPG error: http://us.archive.ubuntu.com precise Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
I have attempted the following, as suggested by several help sites:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5
I have run sudo apt-get upgrade as well as rebooted. However, I still get the same signing key error.