Why am I getting #VALUE in the cell when E15 is blank in the below formula?

=IF(E15="","",IF(E15="1x",(SUM(G15*1)),IF(E15="3x" ,(SUM(G15*3)),IF(E15="6x",(SUM(G15*6)),IF(E15="10x ",(SUM(G15*10)),0)))))*4.5

I thought that the IF(E15="","" portion of the formula would keep that from
appearing.

Any ideas?

Thanks much!
--
Krista