Ah yes, good catch. I was going for dense and elegant, and missed that case.

This one counts the string "True" along with the boolean True.

I don't think this will work if you have codes that have duplicate characters, but as far as the test goes, good job!

Unfortunately the test cases do not match the description. There shouldn't be any negative input values at all.
That said, it seems, that with the current (haskell) tests it is expected to treat negative values like any other value. Zero is only expected if one of the inputs is zero.

I think this kata needs better tests and a more precise description.