You're trying to cast to int something that can't be.

Return nil (or your language's equivalent) for invalid inputs.

I'm getting the same thing. Passed all 8 tests in Python but I'm getting an exit code with error: ValueError: invalid literal for int() with base 10: ''
Can't figure this one out.

(Python)
Passed all 8 test but still getting this:

Traceback (most recent call last):
File "main.py", line 15, in
Test.assert_equals(up_array([]), None);
File "/home/codewarrior/solution.py", line 11, in up_array
num = int(num)
ValueError: invalid literal for int() with base 10: ''

python