Insert formula into a cell doesn't work :S

If got this piece of code:

For x = 5 To 10
Range("D" & x).Formula = "=COUNTIF(PEN!$G:$G;$B" & x & " & \"CO\")"
Next x

Should result in:
=COUNTIF(PEN!$G:$G;$B5 & "CO") in cell D5
...
=COUNTIF(PEN!$G:$G;$B10 & "CO") in cell D10

But it doesn't do the trick... someone who knows how to make it work?! Thanks in advance!!

Steven