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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
The test case comparison only works when the outcome of your function is a string. In your case it is a 'NonType' object. Since "NoneType" object doesn't have certain attributes (e.g.: it doesn't have 'replace' attribute), the evaluator fails to run the test case comparison.
In a nutshell, please make sure your function returns a string
I have the exact same issue