.

I adjusted the reference solution so that it doesn't modify the existing hash.

It doesn't test for not doing this, but almost no kata does that, though it could be a BP for languages like Ruby which have bang annotation for mutating functions?

.. takes a .. hash .. and returns that hash ..

( my emphasis )

Maybe the function should have a different name ( sounds like a very useful convention BTW! ), but that's not the original issue, and I'm not sure it qualifies as an issue ( it might. I don't know how strict that convention is generally adhered to ).

The description is very clear that the hash should be modified in place.

Should this be closed, or not?

If the Hash is modified in place the method name should end with a !, according to convention.

Maybe the problem was changed, but this wasn't a problem for me.

It means non-alphanumeric. A couple of the tests contain non-alpha characters.