An injection needle assembly (50) for an injection device, comprising: a) a needle cannula (3) attached to a needle hub (4) and defining a pointed tip at a free end (3a, 3b), and b) a needle cover (10, 10a, 10b) forming an axially extending elongated flexible enclosure accommodating the needle cannula (3). The needle cover (10, 10a, 10b) is configured to axially collapse and become penetrated by the needle cannula (3) when a penetration force is applied to the needle cover. The needle cover (10, 10a, 10b) defines a shaft section (11a) and a bulb section (11b). The shaft section (11a) encircles the needle cannula (3) and extends axially from the needle hub (4) to the bulb section (11b). The shaft section (11a) comprises wall areas of minimum wall thickness t1 whereas the bulb section (11b) comprises wall areas (17a, 17b) having a wall thickness t2 larger than said minimum wall thickness t1. The shaft section (11a) comprises one or more reinforcing ribs (16), each reinforcing rib extending non-parallel to the needle axis so as to at least partly encircle the needle cannula (3), wherein each reinforcing rib (16) divides adjoining annular collapse regions (15) of the shaft section (11a).
