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