non contiguous range in SUMPRODUCT

I'm having trouble using a non contiguous range in SUMPRODUCT. I'm testing for the presence of "x" in a range of cells: C13,G13,K13,O13,S13,W13
with this formula
SUMPRODUCT(0+(C13,G13,K13,O13,S13,W13="x"))
#VALUE! error.

However, if I just work one cell like SUMPRODUCT(0+(C13="x")), I get the correct response.

how can I get around this?