The referendum, originally scheduled for 1992, was intended to give the local population of Western Sahara the option between independence or affirming integration with Morocco, but it quickly stalled. In 1997, the Houston Agreement attempted to revive the proposal for a referendum, but likewise has not met with had success. , negotiations over the terms of any potential referendum have not resulted in any substantive action. At the heart of the dispute lies the question of who qualifies as a potential voter; the Polisario has insisted on only allowing those found on the 1974 Spanish Census lists (see below) to vote, while Morocco has insisted that the census was flawed by evasion and sought the inclusion of members of Sahrawi tribes which escape from Spanish invasion to the north of Morocco by the 19th century. Consequently, both sides blame each other for the stalling of the referendum, and little progress is likely to be made in the near future.