the random testing is broken.

Testing for 16 9 4 20 18 12 8 1 7 14 3 13 10 2 17 5 19 6 15 21
It should work for random tests too: 1 should equal 11

UPD:
I was converting string to numbers.
So this is my issu
Nevertheless the description can be more clear for this case.
Or use in random test non numeric characters.

Use print in your code.

Try more and you can add print to your code to find input values for examples where you fail.
Had the same problem and then found that I have missed some cases.
First case don't filtering out non alphanumeric characters.
Second case where char in text string appearing more times then in original.