This invention is directed to a closable element for bag, which has a slider mounted on the fastener strips which is movable longitudinally along the fastener strip to progressively separate the rib from the groove to open the bag, and is movable in the opposite direction to progressively return rib to a retained condition in the groove and create a leak-proof seal. A cradle is formed near one end of the fastener strip which is sized, constructed and arranged to receive and retain the base of the slider when the fastener strip is sealed to prevent leakage from that end of the fastener strip. The cradle preferably has a length greater than the length of the slider so it is retained therein and is curved in its interior configuration. In a preferred embodiment, the cradle has a tab which is also curved and extends angularly from the cradle for abutment against and retention of the slider within the cradle.