Disclosed is a transmission lock for a superposition steering system of a motor vehicle. The transmission lock is for locking a rotatable transmission element associated with the transmission or interacting with the transmission with respect to its rotational movement. A clamping element, such as a clamping roller, is used to positively lock, by way of a contour of an inner peripheral surface of an outer ring and a contour of an outer peripheral surface of an inner ring, the inner ring with the outer ring in opposition to an elastic force, and that the clamping roller is retained in an activation cage that can be locked by way of an actuatable locking member.