.

User solutions that return wrongly typed values are just bad, and the error message is quite explicit.

Maybe if it were an 8kyu kata I'd expect tests to mollycoddle solver, but this one, a 6kyu - well, not really.

Other opinions (!) are doubtlessly available.

I would keep the issue open. Tests which crash on an unexpected value returned by user solution are just bad.
Tests should properly inspect returned value for correctness and not just crash.

Not a kata issue. The error is pretty clear about the fact that your function returns nothing.