If both strings have the same length, both your vars shorterString and longerString are s2. So your code ignores s1 content and you think s1 and s2 are equal. Always when debugging print the input values as they were (without any processing from you), it'll help you next time.

Random tests strings (because there is no fixed test with that input) are randomly generated, and it would be very very rare both string were exactly the same. I've checked the control function and if the strings were those, the expected output would be 'ghjlqrtuvxz'. Try posting your code marking your post as having spoiler content.

When reporting a problem, state the language (more if the kata has several like this one) so it can be checked.