Has anyone use the Dandelions - AF confirm chip before?

I was using this af confirm chip with Nikkor 28mm f2.8 ais on my 40d. I took a focus test shot with tripod at f2.8 by manual focusing with live view first. I found that when I stopped down to say f4, I cannot take the shot because the af confirmation is not there...does anyone knows what the reason for this?

I can't think of the reason because everything other than the physical aperture remained the same. I could take the shot if i disconnect the af chip.