Assertion failed: Guid.IsValid()

I’m having a very strange error pop up:

Assertion failed: Guid.IsValid() [File:D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.9\Engine\Source\Runtime\Engine\Private\Animation\Skeleton.cpp] [Line: 82] 

This error did not occur while in the editor, and only occurred when I hit launch. Now that I hit the launch button, it now occurs before the editor is able to open. I’m not finding much information about Guid with relation to skeletons or what it means. The function itself is rather unhelpful:

return (A | B | C | D) != 0;

Any ideas of what is causing this and why it only started occurring after I packaged?