Group Sort on a Sum of a Sum Formula

I'm trying to Group Sort on a sum formula but the formula is not appearing as an option in the Group Sort Expert dropdown. The formula I want to sort by is a 'sum-of-a-sum' formula. This is it:

Sum ({@JanM}, {@AdjName})

+Sum ({@FebM}, {@AdjName})

+Sum ({@MarchM}, {@AdjName})

+Sum ({@AprilM}, {@AdjName})

+Sum ({@MayM}, {@AdjName})

+Sum ({@JuneM}, {@AdjName})

+Sum ({@JulyM}, {@AdjName})

+Sum ({@AugM}, {@AdjName})

+Sum ({@SeptM}, {@AdjName})

+Sum ({@OctM}, {@AdjName})

+Sum ({@NovM}, {@AdjName})

+Sum ({@DecM}, {@AdjName})

The formula itself works fine, CR just wont let me group sort by it for some reason. These other formulas do appear as options in the Group Sort Expert dropdown: Sum {@JanM}, Sum of {@FebM}, Sum of {@MarchM}, etc....(all of the months), just NOT the total Sum formula which is a sum of all the monthly formulas. Help?!

Thank you!!

Rich