Lync 2013 Client Cert warning

Hello All,

We have Lync Server 2010 Standard running with Lync 2010 Clients.  It consists of a single front end and back end along with TMG as reverse proxy.  Everything is working fine with Lync 2010 clients.

However while testing with a Lync 2013 client we receive the below cert warning.  After clicking ok and "always trust server..." the clients connect with no issues.