Global Math Expression field ignores "format plural" settings

I've added a simple "Global: Math Expression" field to my view. All it does is subtract one integer from another (using field replacement tokens) and displays the result. I checked the 'Format plural' checkbox and wrote in my singular and plural text. However, for both 1 and @count strings, no rewriting of the value was done. It always just displayed the number calculated without any of my format_plural() text.

Attached patch fixes this issue.