VLOOKUP and IF in Excel

an excel cell:

=VLOOKUP(A11;datax;17;0) 

Function returns a integer -- But then if a try:

=IF(VLOOKUP(A10;datax;17;0) > 0; VLOOKUP(A10;datax;18;0) ;  VLOOKUP(A10;datax;17;0) )

It returns #REF!

Why?