Did you have time to look at the example with this error?
For completeness' sake: The bug probably occurs when using "instance_create_depth" which adds a new layer for that specific object and Muffel can't find it (due to limitations of GM). I have not heard back from Dave if this resolved his problem yet, but I would be surprised if not. If anybody else has the same problem, feel free to contact me! Here is the fix if you want to try it for yourself:
Either use "instance_create_layer" instead of "instance_create_depth" in your own script, or do the opposite in the Muffel script (replace "instance_create_layer(0, 0, layer, obj_muffel)" with "instance_create_depth(0, 0, 0, obj_muffel)")