An assembly for a drug delivery device (200) is presented which comprises a housing (24) having a proximal end (25) and a distal end (26), a cartridge, a plunger (19) which is movably retained in the cartridge (14) and a piston rod (6). The assembly further comprises a rod displacement feature (33). The assembly is configured such that, in an initial state of the assembly, the piston rod (6) and the rod displacement feature (33) are arranged such that movement of the rod displacement feature (33) from the first position to the second position is transferred to the piston rod (6) such that the piston rod (6) is moved with respect to the cartridge (14) and the assembly is switched from the initial state to a primed state. In the primed state, movement of the rod displacement feature (33) from the first position to the second position is not transferred to the piston rod (6).