It is indeed an issue. It should be fixed.
The error comes from the test, not your code.
You can modify your code to accomodate, but it really should be fixed in the kata's tests.

If in the function definition you write
"def arr(n=0):"
instead of
"def arr(n):"
then it will run just fine.

But this is clearly out of the scope of the exercise, as one should not have to modify the function that's given to one in such exercises...

Your code should handle that test too, it's not a problem with the test.

If you are not sure, you should not raise an issue. The kata runs fine in your language (python), and please note it has over 2K solutions. If you can prove it has an issue, provide the test case, expected output, your output, and why the problem arises.