Sorry for my late reply on this. Class ended and I forgot to follow-up. I think it may have been specific to the data that I had displayed at the time I was trying to run the check. When I run the tool on the parent (an SDE Oracle geodatabase) I do not get an error. I have also just tried running it again on another checkout with the same data and did not get the error. Prior to running reviewer we went through the process of data loading, so it might be something with the data that got loaded. Unfortunately I am unable to recreate the error at this time. I am teaching the class again next week though and will let you know if it happens again.