Baking Lightmaps completely from CLI not working - HALP!

Hey guys, we’ve been banging our head against a wall here for a couple of months and haven’t found a solution to this problem. Whenever we try to bake lights in a scene, even the simplest empty scene you can imagine it quits with this error: Assertion failed: FunctionInputs*.ExpressionInput [File:D:\Build++UE4+Release-4.12+Compile\Sync\Engine\Source\Runtime\Engine\Private\Materials\MaterialExpressions.cpp] [Line: 7309]

We are attempted this on a workstation running Windows Server 2012, Intel Xeon E5-2670 with a Quadro K620 and the latest Nvidia Grid drivers.

Has anyone had any luck baking from the command line, particularly from an amazon server? If so how did you overcome this issue?