A lock for a motor vehicle, in particular a side door lock, comprising a locking mechanism having a rotary latch and at least one locking pawl, an actuating level chain with at least one actuating level and a release lever, wherein a blocked locking mechanism can be unblocked by means of the release lever, and a coupling element between the actuating lever and release lever, wherein the coupling element can be actuated by means of a mass inertia element which is mounted in the motor vehicle lock, and wherein the mass inertia element can be stored in the motor vehicle lock by means of a plastic mandrel.