A printhead assembly with a printhead integrated circuit (IC) and a shut off (138) valve that has a valve body (200) defining an ink inlet (202) for connection to an ink supply, an ink outlet (67) connected to the printhead IC. A moveable valve member (212) is biased into sealing engagement with the valve seat (216) to provide a fluid seal between the ink inlet and the ink outlet. An actuator (204) for unsealing the valve member from the valve seat upon energizing and re-sealing the valve member to the valve seat when de-energized. The valve protects the ink in the ink supply from contaminants that can migrate up the ink line during shut down periods. The valve member is constantly biased to a closed position and so seals the ink supply from the printhead IC as a default condition even in the event of a power failure. The bias is strong enough to provide the fluid seal so that the seal is not compromised when the pressure difference between the inlet and the outlet is small.