• lacking sample tests.
• assertion messages aren't really helpful. => show one possible output to the user or at the very least give the minimum length expected

I didn not read the name of the Kata (usually it is not necessary), so did not understand that I have to to find bigrams only. I was looking for all possible сombinations... :( It was sad. Maybe you could add one more example ('abc'?) to description to make it clearer?