The present invention relates to a method of assembling a drug delivery device (100, 200) comprising an expelling mechanism for providing a force on the piston (114, 214) of a held cartridge (13, 113, 213). The method includes the steps of a) providing a cartridge (13, 113, 213) comprising a cartridge body having a distal outlet being closed by a penetrable septum (17) and having a proximal open end closed by a slidable piston (114, 214), b) providing a piston rod (120, 220) for transferring a force to the piston, c) arranging the cartridge and the piston rod along an axis, d) providing a monitoring arrangement (A, B) configured for detecting a change in geometry of the septum (17), and e) providing relative axial movement between the piston rod (120, 220) and the cartridge body for causing the piston rod to move the piston (114, 214) towards the septum (17) with the effect of deflecting the septum until the septum assumes a predetermined geometry change. The invention further relates to a drug delivery device assembled according to the method.
