Showing a text instead of evaluating the formula using firmulaR1C1

I have this code:

Cells(PathFinalRow + 1, 6).FormulaR1C1 = "=SUM (R2C:R[-1]C)"

But it doesn't evaluate the formula and just writes =SUM (R2C:R[-1]C) into the cell.
The cell format is General and Show formula is off. I have no idea what is wrong. I've just seen a similar code on a reference book but it's not working for me!

I would appreciate any help!