A month ago I announced a new version of IsWiX with support for WiX v4.0. At the time I wrote the warning:
I tested all of the designers but there is a chance that one of my refactoring's, well, sucked. I'm sorry, I don't have unit tests to know for sure. If you find a bug, please report it and revert to IsWiX 3.0. I will follow up with an IsWiX 4.0 fix in a matter of days possibly hours.
Sure enough, one of my updates did, cough, "suck". It seems I didn't test all of my designers, rather only the ones that appeared. I missed the fact that one of them wasn't showing until I needed it to do some work for a client. I opened up IsWiX and went to define a custom table and was really disappointed that it wasn't there. So I followed my own advice and downgraded to IsWiX 3.0, kept calm and carried on. Eventually I had some time to go look at it and sure enough the IsValidContext() member simply needed to be refactored.
The new and improved IsWiX can be found here.