Sorry, my mistake the "=" was not moved when I adjusted the formula
=if(sum(ap155:ap184)=0,0,sumproduct((ap155:ap184>0)*ap155:ap184)/sumproduct(--(ap155:ap184>0)))
You don't have an error to detect for so this is unecessary, what you did have is a crap formula (I appologise again), I should have...