Guess ya' never heard of the Great Western Schism, huh? How long was the papacy in dispute, 39+ years was it not? And of course YOU know the maxim: a disputed pope is no pope.
And just how long was the See vacant before the election of Pope St. Celestine V?
During the schism, there was a true pope as well as the antipopes.
http://www.angelfire.com/ms/seanie/papacy/schism.html The last validly elected Pope before the schism was Pope Urban VI, who was elected in 1378. The list of true pontiffs during the time of the Great Western Schism was : Urban VI, Boniface IX, Innocent VII, and Gregory XII. Gregory XII called the Council of Constance in 1414 on condition it accept his authority to call it and then resigned as promised. The schism was ended with the election of Pope Martin V.