A symbol `mp` here is correct, as Amit notes below, and `m'` or `mp` is typically used for this. It is due to having to sum over allowed `m'` values as determined by `j`, but with a symbolic `j`, it must return a Sum.

As a side note, @smichr do you know if there is a way to create a test when a Dummy is returned? I remember doing some weird things in spin.py that really should've been done with a Dummy, but I couldn't figure out how to test the output.