Okay, I know when it happens now.

I included an invisible, passable block to encompass the models as a reference to the size of the map entity and because models in map entities are not rendered in WED. If there is an invisible block in a map entity it will throw an error if you do not also include a visisble block. This seems uneccessary, so can this requirement be removed?

I will send you an example if it helps.