I got the same situation and finally I found the reason:
You should consider text="" and text="And a Egg"('A' 'E' 'I' 'O' 'U' are included).
examples all use text="this a test!" made a trap！

Hi IcodeU,

your solution doesn't work for the large shifts. That's the problem.

For the above tests the shifts 127 and -127 are used. Your solution crasches with such shifts.

You should rework your code.

Hi IcodeU,

post your code here please and mark it as spoiler.