Character Converting Into Number In 2D Char Array?

I'm trying to convert all letters in a 2D char array into number. As results, I want a = 0, b=1, c=2, ... z=25.

Then, I tried this as part of my code:

char [][] letters = {{'i', 'u'}, {'a', 'g'}, {'e', 'k'}};
for (int i = 0; i < letters.length; i++) {
for (int j = 0; j < letters[i].length; j++) {
if (letters[i][j] >= 'a' && letters[i][j] <= 'z') {
letters[i][j] = (char) ((letters[i][j] - 'a') + '0');
}
}
}

The result of my code is not what I expected before.

From 'a' to 'j', it worked well. But, from 'k' until 'z' it didn't print expected number.

What's wrong with my code and what is the correct way to fix it?