Edited Unassigned: Setting Formula Is Doing Strange Things [9345]

We are setting multiple formulas throughout our process and all but 1 is working. I make this call:

ws.Cell(13, 3).FormulaR1C1 = "SUM(C8:C12)"

and it shows like this in Excel:

=SUM($H:$L)

If I manually key in the formula it works just fine and, again, the other SUM formulas are working just fine. See attached example.