AppSync JS resolver do not support the "in" operator?

Hi AWS folks! I've just been stuck on a failing CloudFormation deployment for a while. One of my AWS::AppSync::FunctionConfiguration resources was failing with the message The code contains one or more errors.. It is one of the newer JS resolvers types (as opposed to VTL). After some trial and error, I determined that it was because I was using the in operator. However, the AppSync docs do not state that this operator is not supported. Perhaps someone can confirm this and, if I am correct, update the docs?