Help wanted with a mysterious Dialyzer error

I’m getting a mysterious Dialyzer error and I’m hoping that someone can help me figure out what’s going on and resolve the problem. As usual, constructive comments and questions are also welcome.

The original project uses Phoenix, but that isn’t relevant to the problem. It also has a pretty large code base, which I can’t expect anyone else to examine. So, I’ve extracted the problematic code into a pair of (much smaller) Mix projects located here.

-r