The problem you have is not that your subscripts are not logical, it is that either they're not positive or they're not integers. I'm fairly certain, even with the very limited info you've provided, that converting to logical won't give you the result you want.
- make sure that X, Y, i and j are integers
- make sure that X > i and Y > j
- and, you never know, make sure that you haven't got a variable called sum that stops you from using the sum function.
And on that last point, don't use mean as a variable name since it prevents you from using the mean function.