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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.
Are you checking for 'null' or an empty string?
That might be where you're failing, as the test cases (I don't think) test for those situations. (null being the function not having a value in the string portion; empty string being an empty string entered... i.e. encrypt( ,1) is null because there is nothing entered, where as encrypt('',1) is having an empty string for an argument.