Currently, uint32 is not supported in Blueprints, as reported here:
Blueprints can edit variables of type int32, uint8, and float. All others can be seen by Blueprint but not edited outside of code. A BlueprintImplementableEvent won’t be able to return uint32.
Yeah, after trying this I realized that uint32 isn’t supported - however my concern was more fact that error message produced was totally uninformative so it took a bit to figure out exactly what was going on. For anyone who doesn’t realize unsupported types, it definitely doesn’t point them in right direction!