Formula does not display in subtable column: glitch or error?

Hello

I have a formula (see below) that appears to work perfectly, but does not display in the subtable column. Is this a glitch or an error on my part?

Either way, does anyone know a way around it?

Many thanks

Ben

Formula:

if “No. of seeds” = null then
null
else
“No. of seeds” / Weight
end

(without the formula there are many zeros in the column, which I want to avoid so i can calculate an accurate average of multiple results)