Sort by another column error

Hi

I have categories that I want to sort. The data looks like this:

To populate the sort order column, I wrote the following formula:

Sort Order =

    IF ( [Category] = "A", 1,

    IF ( [Category] = "C", 2,

    IF ( [Category] = "B", 3)))

Then, I tried to use the "Sort by Column" tool to sort the Category column by the Sort Order column, but the following error message occured:



Anyone know how the reason for this and how to resolve?

Thanks!