The invention relates to a needle unit (2) for connecting a drug delivery device to a medicament reservoir (1), the needle unit (2) comprising:- a hollow cannula (3) having a sharp proximal tip (3.1) for piercing a reservoir wall (1.1) of the medicament reservoir (1),- a stop member (5) which is axially fixable to the cannula (3),- a sleeve (4) disposed on a section of the cannula (3), the sleeve (4) comprising a proximal section (4.1) and a folding section (4.3),wherein in an initial state the folding section (4.3) is substantially concentrically arranged with respect to an outer surface of the cannula (3), wherein the folding section (4.3) is adapted to expand radially outwards by moving the proximal section (4.1) towards the stop member (5).
