Null workset?

I have a simple graph that shows empty worksets so I can delete them. On a recent model I received I’m getting an error, a couple worksets are showing up as null and not empty like others. What is happening?
The node warning says: Warning: Select.ByWorkset operation failed.
The type initializer for ‘Revit.Elements.DirectShape’ threw an exception.

What came up for me was:
Was the model you received checked out properly?

That was it! Usually when I run this all worksets are checked out, but they weren’t today. Normally I open as detached, not sure how i missed that today.