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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.
Really challenging kata for me, had me so frustrated at times. Anyone who's stuck, the other comments here will guide you in the right direction.
This had me stumped too for a while. If you're still having trouble with this kata:
We know from this error that your solution is failing because Ruby is trying to call a method on NilClass, but that specific method doesn't exist for NilClass.
What being passed in to your method as an argument could belong to NilClass?