WSUS synchronization error - SCCM 2012

I have a virtual lab containing a WSUS server and an SCCM server.  I setup the WSUS server as the software update point in SCCM, but I am continuously having issues synchronizing.
 
As they are in a lab, they do not have internet access.  I gave the WSUS server access after install to download the latest updates, but after that, put it back on the lab network.  SCCM is only trying to synchronize the already-downloaded updates from WSUS, correct?  In other words, the lack of internet access should not adversely affect synchronization with the WSUS server?
 
The error is;
Type: Mileston
Severity: ERror
Message ID: 6703
Process ID: 2044
Thread ID: 1600
Description:
WSUS Synchronization failed.
Message: WSUS server not configured.  Please refer to WCM.log for configuration error deatils.
Source: CWSyncMgr::DoSync.
The operating system reported error 2147500037: Unspecified error
 
Any ideas?