I think I've seen this once before, too.
If you can attach an ontology that throws this error to this issue, that would
be of great help.

I get a dialog with this text in it, but I don't know how to track down what is 
missing. It would be helpful to at least say something in the dialog. It might be 
a good idea to include it in the species validation reports. Paradoxically an 
ontology with this error still validates as OWL-DL, but I though (perhaps 
incorrectly) that one needs to declare what sort of thing each entity is in order 
for it to be OWL-DL.