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. <EOS>