There's an issue with the Elm compiler documented here that results in the error message:
Map.!: given key is not an element in the map
Though it is not resolved, there are a few things you can do to get around it for now:
Arrayin a custom type (use
Array in a custom type was what led me to the error, and substituting for
List made it go away.
Looking forward to updating this post when the issue is resolved!