MSExchange OWA Event 139 - HTTP 500 Error when accessing OWA

I'm having a strange issue when attempting to access OWA.  I receive a HTTP 500 error, however, this is what's logged in the Application log:

________________________________

Event 139, MSExchange OWA

There was an error setting up WacConfiguration. Wac will be disabled and we will fall back to stellent. The WacUrlHostName was invalid. Expected a valid Uri. Actual value was ''. Value read from 'OrganizationConfig'

________________________________

This is an Exchange 2013 RTM install on Server 2012 with both Client Access and Mailbox roles.  I can access ECP just fine and I double checked to make sure the server was properly configured for external access.  Any thoughts?