Why error catcher doesn`t work with this formula?

[ABSTRAKTUS]

Hi all! Errors drive me crazy! Especially when I can` get rid of them. So, I have a formula, and error catcher =IF(ISERR( does`nt work. Excel says you have a mistake in your formula. But I am sure enough I don`t. There is a formula :

=INDEX(OFFSET(Stafflist!$B$5,MATCH(VLOOKUP($C$5,$Q$17:$R$20,2,0),Stafflist!$B$5:$B$163,0),1,ROWS(INDIRECT("Shift"&VLOOKUP($C$5,$Q$17:$R$20,2,0)))),SMALL(IF("O/T"=OFFSET(Stafflist!$B$5,MATCH(VLOOKUP($C$5,$Q$17:$R$20,2,0),Stafflist!$B$5:$B$163,0),MATCH(MAIN!$G$5,Stafflist!$C$5:$AJ$5),ROWS(INDIRECT("Shift"&VLOOKUP($C$5,$Q$17:$R$20,2,0)))),ROW(INDIRECT("1:"&ROWS(INDIRECT("Shift"&VLOOKUP($C$5,$Q$17:$R$20,2,0))))),""),ROW(A5)))

The formula works fine, but when there is no value to return to that cell, it comes up as #NUM error. Conditional formating is not for me. These dark triangles in the corners of the cells drive me crazy as much as errors do. How do I get rid of the error in the cell if there is no value to return?