An interlock assembly for a door latch mechanism of a door in a container for bulk material that has an interior lined with a membrane for holding fluid. The interlock assembly includes a plunger in an aperture formed through the door, which moves between an extended position, where the plunger extends into the interior of the container, and a locked position, where the plunger is urged outwardly from of the interior of the container. A spring is positioned to apply a force to urge the plunger to the extended position. A lock member is connected to move with the plunger between the extended position and the locked position. The spring force is selected to enable the membrane, under static pressure of fluid within the container, to urge the plunger to the locked position, thereby engaging the lock member to interlock against operation of the door latch.