[X4U] Excel problem

[Paul Moortgat]

In colon 2 is the number 2
In colon 3 is the number 2
In colon 8 there is the text OK
In colon 9 that when 8 contains OK I need to add the values from colon 2 and 3 which should give 4.
If colon 8 is not OK the cell in 9 is 0 (zero)

For this I use the formula in colon 9:  =IF(RC[-1]="OK",SUM(RC[-7]:RC[-6]),0) which always gives me an error for OK.
I tried 'OK' and "OK" and a plain OK.  Nothing helps.  I get the message that's there a problem with the OK description in the formula.
What do I wrong?  I use the R1C1 way in version 11.

Paul Moortgat