Can we be held responsible for an ex roommate's light bill?

we're being held responsible for our ex roommate's electric bill , when we went to have it put in our name they said unless we pay his preexisting bill we could not have any service, my wife did sign an agreement to do so but upon looking at the agreement it was left blank where it was supposed to be notarized it has all the places where it was supposed to be notarized but all was left blank does that mean the agreement is not a legal doent?