Drug delivery device adapted to receive a cartridge, comprising drug expelling means with a piston driver comprising a distal end adapted to abut and axially move the piston of a loaded cartridge to thereby expel an amount of drug from the cartridge, and a motor assembly for moving the piston driver. The piston driver distal end comprises flexible centring means adapted to frictionally engage a cartridge inner surface proximally of the piston, thereby centring the cartridge and the piston driver distal end relative to the each other.
