A closure system including a filler neck, a guide socket arranged inside the filler neck, a closure cap to be introduced between the filler neck and the guide socket and which is fastenable to the filler neck and an annular seal to be received in an annular groove of the closure cap and which is secured to an inner face of the filler neck. The annular seal is arranged between the filler neck and the guide socket such that a nozzle placed between the filler neck and the guide socket does not reach thereto.