I was also getting errors outside of my code initially, as under certain circumstances I was returning null in my frequencies method - javascript
How I found my problem is by printing every method call and input, try doing that

"I'm getting errors outside my own code" is not very helpful. how do you expect us to know what's wrong ? what language are you using ?

Not sure why, it worked for me. If you want, you can post your code with proper markdown (don't forget the spoiler tag) and I'll take a look.

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Kata works, your problem is not a kata issue.

It's difficult to say what exactly happens without seeing your code, but if I were to guess, you do not handle memory management correctly. It's not said explicitly, but C version requires you to return a freeable string.

Some people downvoted your post. I think it is because these people saw that the Python kata was passed by 3672 people (see the top of the page) so if you fail the tests it is not because of the kata but because of something wrong in your solution. Sorry! Succeeding on your computer is good but you do succeed at CW. I don't think that the difference can be explained by the difference of Python versions.