A power-assisted steering with a superimposing gear mechanism enables safe steering behavior independently of the type of transmission used. A mechanical lock is mounted at a part of the transmission, preferably at the screw itself. In the event of a malfunction, this lock acts in addition to the deactivation of an electric motor and reliably prevents it from rotating further in that a locking pin engages elongated hole-shaped grooves. The shape of the grooves has the advantage that the locking pin need not be positioned very precisely. Allowances are thus made for installation tolerances. The development of noise is also reduced since it is very improbable that the locking pin, before it is locked in one of the grooves, would first of all come into contact with an intermediate bridge and thus produce an unpleasant clacking noise. In addition, a diagnostic circuit may be provided which checks the function of the lock before the vehicle is started up.