I'm doing something wrong and can't figure it out. Can anybody help me?

Heres what I want: If user enter value in cells A1 through A5, I want cell
A6 to return €œMust be filled in€�

Heres the formula I put in A6: =IF(A10,"Must be filled in!","") and it
works, but only for one reference cell.

What I want is a range of cells (A1:A5). So I enter in A6: =IF(A1:A50,"Must
be filled in!",""). And I get a #VALUE! Error in A6.

Help!
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Merci!
Bonzai