add two sums and return blank cell when sum value totals zero

Hi, I am trying to find a formula to add the value in each row of column Z to the corresponding row in column AQ and display the results in column AS.

The present formula works fine as long as there is no blank cell in any of the rows Z or AQ. If you look at the cells highlighted red (AS9 & AS11), these are picking up blank cells and instead of showing values of 4 and 16 respectively, they are showing blank cells.

Attached is a spreadsheet which hopefully further explains the issue I have.

If anyone can assist in providing a formula which works, this would be appreciated.

Thanks

Andy

this will leave the cell blank only if both are blanks and then test the each column for a blank

=IF(AND(Z11="",AQ11=""),"",IF(Z11<>"",Z11,0)+if(AQ11<>"",aq11,0))

Thank you all for your help which resolved my problem.