Did you ever get this work? I'm having the exact same problem in C. I tried returning NULL as the function's return value as the test indicated, but that didn't work. Then I turned the bad entry into a 0 (NULL), but that didn't work either.

hold up am i suppose to replace the specific part of the array with null instead of invlidating the whole sequence... cause i'm "returning NULL;" upon any one of them being invalid.

but how is it my own bug when all i'm doing is "return NULL;"?
"return NULL;" is called upon the number having more than one degit ie. > 9 and when its negative ie < 0
i even print tested each section, and 33 & -1 does get handled correctly and is passed off to the "return NULL;"
and ik "return NULL;" is called as the print statement afterwards isn't called.
so i don't see how it is my bug-

Your solution handles some specific invalid inputs incorrectly. Read the description carefully and verify whether your solution handles all possible invali inputs.

Having said that, I agree that feedback of tests could be improved.

Bugs in your solution are not a kata issue.

The test case for my invalid inputs are working, but when i "return NULL;" i can't seem to pass the invalid inputs test, the return is nested within the if statement and seems to be accessible, it does also end the function as no other print statements after return does work.

So what's going on? my language I'm using is C.

Conclusion: return NULL isn't working in C for invalid inputs test