A medicament delivery device comprising: a first sub-assembly; a second sub-assembly including a proximal housing; and the first sub-assembly (8 comprising a plunger rod which is biased; a main body arranged on the plunger rod; and a coupling member movable relative to the main body; characterized in that: the main body comprises a first locking element (825; 829), the coupling member comprises a second locking element, wherein the first locking element (825; 829) and the second locking element are configured to be releasably engaged with each other such that the coupling member is prevented from moving relative to the main body, and wherein the first locking element extends in an axial direction from the main body, then extends radially outwards in an arch, and then extends radially inwards so that a free end of the first locking element is engaged with the second locking element to prevent the coupling member from rotating relative to the main body, and wherein a distance between the arch and a central axis of the main body is larger than a distance between the central axis and other portions of the first locking element, and also larger than an outer radius of the coupling member, and wherein the housing is configured to interact with the first locking element (825; 829) such that the first locking element (825; 829) and the second locking element are configured to be released from each other upon assembly of the first sub-assembly with the second sub-assembly by an inner surface of the housing disengaging the first locking element (825; 829) from the second locking element by bending the first locking element (825; 829) inwards. a first sub-assembly; a second sub-assembly including a proximal housing; and the first sub-assembly (8 comprising a plunger rod which is biased; a main body arranged on the plunger rod; and a coupling member movable relative to the main body; the main body comprises a first locking element (825; 829), the coupling member comprises a second locking element, wherein the first locking element (825; 829) and the second locking element are configured to be releasably engaged with each other such that the coupling member is prevented from moving relative to the main body, and wherein the first locking element extends in an axial direction from the main body, then extends radially outwards in an arch, and then extends radially inwards so that a free end of the first locking element is engaged with the second locking element to prevent the coupling member from rotating relative to the main body, and wherein a distance between the arch and a central axis of the main body is larger than a distance between the central axis and other portions of the first locking element, and also larger than an outer radius of the coupling member, and wherein the housing is configured to interact with the first locking element (825; 829) such that the first locking element (825; 829) and the second locking element are configured to be released from each other upon assembly of the first sub-assembly with the second sub-assembly by an inner surface of the housing disengaging the first locking element (825; 829) from the second locking element by bending the first locking element (825; 829) inwards.