Should be fixed now.

Ruby.

What language are you using? That is definitely an issue.

Your test suite (random tests) is probably broken:

Testing for cuwlkypvhkno and rwnqcxrmphlfkftovqt
It should work with random inputs too - Expected: true, instead got: false

The first string contains "u" and the second doesn't.

The second string contains "x" and the first doesn't.

So you cannot build any string using letters in the other.