Formula ignored if missing value

Morning,

I'd appreciate any help you can give. I am using the following formula:

=IFERROR(VLOOKUP(Achieved!AE4,Lookups!$O$1:$P$9,2),0) - IFERROR(VLOOKUP('FFT Target'!AE4,Lookups!$O$1:$P$9,2),0)

The problem I'm having is that I want the result for this formula to leave the cell empty if either of the Looked Up cells are empty. Any ideas please?


Many thanks