I have this formula

=IF(ISNA(VLOOKUP(A17,'Data-FSList'!GEAC2000IntlFSList,2,FALSE)) =TRUE,
"DONOR NOT VALID",
VLOOKUP(A17,'Data-FSList'!GEAC2000IntlFSList,2,FALSE))

on cell B17.

When the user selects a value in A17 from the drop down, it goes to sheet
'Data-FSList' to retrieve a value. If the value is not there it comes back
in field B17 'DONOR NOT VALID'

However when A17 contains no value from the drop down and is blank I do not
want 'DONOR NOT VALID' to show in cell B17.

Is there a way to add to the formula above if A17 is blank then B17 is blank

thanks