I guess the comment that I added to 2686 didn't actually get submitted like I thought, but I did move it to core successfully. Anyways, the reason was because I was going to fix this in the common convertor code, if the enum passed in is null, just return null, instead of throwing a NPE. Having a null enum generally is a valid operation. So, revert this change, and I'll change the EnumConvertor class right now.