A tonneau cover system is provided for a cargo box of a pickup truck. The cargo box including a sidewall having an inboard side, wherein the inboard side extends downward and includes a lower edge. The tonneau cover system comprises a framework assembly positionable over the cargo box of the pickup truck and a bed rail extender having an engagement surface. The bed rail extender is adapted to attach to the inboard side such that the engagement surface is spaced downward from the lower edge of the inboard side. A clamp assembly is operably coupled to the framework system. The clamp assembly has an engaging member selectively positioned between a latched position and an unlatched position. The engaging member is adapted to contact the engagement surface of the bed rail extender in the latched position to retain the framework assembly to the cargo box.