Sumproduct with Sheet as variable

Hi,
I am trying to use a R1C1 Sumproduct formula with a variable sheet, but I got stuck in this problem. I try to use the string "str" as the variable, yet Excel responds with run-time error 1004.
Any hints,please?

Here the code:

For Each sh In Sheets
str = sh.Name
Sheets("Übersicht").Range("C" & 14 + i).Select
    ActiveCell.FormulaR1C1 = _
        "=SUMPRODUCT((R14C='& str & '!R2C7:R71C7)*('& str & '!R1C10:R1C13=Übersicht!R[-3]C)*' & str & '!R2C10:R71C13))"
Next sh

Thank you very much!