How to get Average for Calculated Column that has values but also NIL value

Hi all,

I am trying to get the average of days customers place orders. 

Both my columns "Product Order Per Customer" and "Frequency" are calculated column. As you can see, some rows have values whereas the others don't. I am trying to get an overall average of no of days customers place order but no matter what I do, the value that came back is NaN. Please help!