The descriptions relate to preventing assembly of a fluid filter apparatus unless a filter cartridge is installed, and preventing assembly of a fluid filter apparatus if the incorrect filter cartridge is installed. A mechanism includes cooperation of a movable member and a structure that can move the movable member to ensure that a fluid filter apparatus cannot be completely assembled, for example connecting its filter head to its shell, when there is no cartridge installed inside the shell or when there is an attempt to install an incorrect cartridge. This can protect from an inadvertent or intentional failure to install a cartridge (or correct cartridge) in the fluid filter apparatus.