An assembly for a drug delivery device (1), comprising a housing (2) having a proximal end and a distal end, a piston rod (8) and an indicator (13). The piston rod (8) is adapted to be displaced distally with respect to the housing (2) away from a proximal start position and towards a distal end position for delivering a drug (31). The indicator (13) is adapted for indicating at least two different operation conditions of the assembly, the indicator (13) being moveable with respect to the housing (2) from a first indication position for a first condition of the assembly into a second indication position for a second condition of the assembly. The piston rod (8) and the indicator (13) are configured to mechanically interact for converting movement of the piston rod (8) into movement of the indicator (13). A first detent (21) is provided which is configured to releasably secure the indicator (13) in the first indication position such that movement into the second indication position is allowed. A second detent (22) is provided which is configured to secure the indicator (13) in the second indication position such that movement into the first indication position is prevented.