IfElse Formula help

Hello everyone Slight smile

I have problem with my formula IfElse and i don't why it doen't work... 

So here is my formula :  ifElse(equal($statut_prospect_c,""),"","ACHAT")

I'm trying to parameterize the "OFFERS" field. I want to indicate that if the $status_prospect_c field is empty, the "OFFERS" field containing this formula will take the empty value "", otherwise "PURCHASE".

I cannot execute the formula, i received the message : "The formula must be of typeboolean"

Knowing that the "OFFER" field is of the multiple-choice list type

Can someone point out what I am doing wrong?

Thanks in advance!