I'm getting an invalid AutoCAD DWG or DXF file message.

Situation: You try to import an AutoCAD DWG or DXF file into SketchUp and you get this message:

Error opening AutoCAD import file: Invalid group code.
This does not appear to be a valid AutoCAD DWG or DXF file.

The DWG/DXF file is corrupt. You can verify this by running the Audit command in AutoCAD.