You have some work left. To solve your current problem, figure out why your code crashes for is_merge('', 'a', 'b'). It should return False.

Let me know if you need more hints along the way.

Thanks for spending time with my kata. :-)

Well, there are 600+ valid solutions for Python, so I don't think the tests are wrong. There are some edge cases that are not properly tested, so if anything, they are easier to pass than they should be.

Are you sure you take into account that the number of characters don't always add up? is_merge('a', 'a', 'a') should be False for example, just like is_merge('abc', 'a', 'b').

If you still are convinced that your code is correct, please let me see it and I'll see what I can do.