A combination of a key and a cylinder lock, - said cylinder lock comprising a rotatable key plug having a longitudinal keyway, and a longitudinal profile rib extending along said longitudinal keyway, and - said key comprising a substantially flat key blade having a longitudinally extending profile groove at at least one side surface thereof, - said profile groove having a profile shape being substantially supplementary to said profile rib and being provided with an inner, longitudinally straight portion corresponding to the outermost portion of said profile rib, said inner portion of said profile groove extending along at least a part of said key blade, characterized in that - said cylinder lock comprises at least one blocking element serving to axially block the insertion of an incorrectly cut key blade, said blocking element being movable up and down in said keyway in a cut-out portion of said profile rib, and having a profile section which corresponds to the longitudinally extending groove in the key blade, - said blocking element, in a region of an outer portion of the profile groove of said key blade, is somewhat larger, in a direction parallel to a central plane of the cylinder lock, so that it does not fit into the profile groove of the keyway along the full length thereof, - said key blade is provided with at least one local recess adjacent to said profile groove, said local recess constituting a locally expanded portion of said profile groove, said at least one local recess being wide enough to accommodate an uppermost portion of the blocking element, and being limited to a longitudinally confined portion of the key blade which is confined to the region of the fully inserted key blade in the vicinity of said blocking element and which is shorter than said part of the key blade and allows said uppermost portion of the blocking element to be accommodated therein, and - said locally expanded portion of the profile groove has a cross-section with two opposite, non-symmetrical side walls, which extend in the longitudinal direction of the key blade. - said cylinder lock comprising a rotatable key plug having a longitudinal keyway, and a longitudinal profile rib extending along said longitudinal keyway, and - said key comprising a substantially flat key blade having a longitudinally extending profile groove at at least one side surface thereof, - said profile groove having a profile shape being substantially supplementary to said profile rib and being provided with an inner, longitudinally straight portion corresponding to the outermost portion of said profile rib, said inner portion of said profile groove extending along at least a part of said key blade, characterized in that - said cylinder lock comprises at least one blocking element serving to axially block the insertion of an incorrectly cut key blade, said blocking element being movable up and down in said keyway in a cut-out portion of said profile rib, and having a profile section which corresponds to the longitudinally extending groove in the key blade, - said blocking element, in a region of an outer portion of the profile groove of said key blade, is somewhat larger, in a direction parallel to a central plane of the cylinder lock, so that it does not fit into the profile groove of the keyway along the full length thereof, - said key blade is provided with at least one local recess adjacent to said profile groove, said local recess constituting a locally expanded portion of said profile groove, said at least one local recess being wide enough to accommodate an uppermost portion of the blocking element, and being limited to a longitudinally confined portion of the key blade which is confined to the region of the fully inserted key blade in the vicinity of said blocking element and which is shorter than said part of the key blade and allows said uppermost portion of the blocking element to be accommodated therein, and - said locally expanded portion of the profile groove has a cross-section with two opposite, non-symmetrical side walls, which extend in the longitudinal direction of the key blade.