I have a problem summing over seven cells from a pivot table. The cells are not grouped together so they are not summed by the pivot tables sum function.

I am getting the following error: The specified formula column cannot be entered because it contains more values, references and or names than are allowed in the current file format.

Here is a rough example of what I am attempting to do out of the pivot table:
Using the sum function and selecting the cell to get the value. Sum all values where the 1st column starts with an r. Sum function will handle 7 cells, but not 8???

I am However able to sum 7 groupings together, sum 7 more together and then sum the two cells. That is not really the route I would like to take.

Rough example of table:
r1, 1
g1, 2
r2, 3
g2, 4
r3, 5
g3, 6
r4, 7
g4, 8
r5, 9
g5, 10
r6, 11
g6, 12
r7, 13
g7, 14
r8, 15
g8, 16

Sum formula:

=sum(GETPIVOTDATA("[Measures].[Tonnage]",$B$80,"[DIM Material Group].[Material Group]","[DIM Material Group].[Material Group].&[RM-A-GM]"),GETPIVOTDATA("[Measures].[Tonnage]",$B$80,"[DIM Material Group].[Material Group]","[DIM Material Group].[Material Group].&[RM-DRC-GM]"),GETPIVOTDATA("[Measures].[Tonnage]",$B$80,"[DIM Material Group].[Material Group]","[DIM Material Group].[Material Group].&[RM-DRW-GM]"),GETPIVOTDATA("[Measures].[Tonnage]",$B$80,"[DIM Material Group].[Material Group]","[DIM Material Group].[Material Group].&[RM-MEP-GM]"),GETPIVOTDATA("[Measures].[Tonnage]",$B$80,"[DIM Material Group].[Material Group]","[DIM Material Group].[Material Group].&[RM-MH-GM]"),GETPIVOTDATA("[Measures].[Tonnage]",$B$80,"[DIM Material Group].[Material Group]","[DIM Material Group].[Material Group].&[RM-ML-GM]"),GETPIVOTDATA("[Measures].[Tonnage]",$B$80,"[DIM Material Group].[Material Group]","[DIM Material Group].[Material Group].&[RM-MM-GM]"),GETPIVOTDATA("[Measures].[Tonnage]",$B$80,"[DIM Material Group].[Material Group]","[DIM Material Group].[Material Group].&[RM-U-GM]"))

Any help would be greatly appreciated. Thanks