Sorry, my bad. Figured it out. Cheers.

When I press "Test", it returns "You have passed all of the tests! :)".

However, when I press "Attempt", it returns an error in Random tests.

Take a close look at the expected results of any random test:

expected

[ 'Mn', 'Wc', 'kU', 'Xp', 'yw', 'fx', 'Uf', 'qp', 'F_', 'FH', 'vX', 'DU', 'Py', 'HI', 'DT', 'jO' ]

to deeply equal

[ 'Mn', 'Wc', 'kU', 'Xp', 'yw', 'fx', 'Uf', 'qp', 'FM', 'FH', 'vX', 'DU', 'Py', 'HI', 'DT', 'jO' ]

If you say there's no problem in random tests, how come 'F_' is supposed to equal 'FM'?!

There seems to be a mistake in random tests when you hit "Attempt"