Validation List ignore blanks (formula)

Hi,

I want to create a validation list with value comming from formula and I got blanks line. I have to create a dynamic list, I can have 15 possibility and it came from a lookup formula. When I choose the 15 cell with the lookup formula it show the empty cell.

I did a exemple of my problem. You can check the attachement to understand my problem in the cell C1.

Best regards