The present invention relates to a mounting aid (400) for attaching a supplementary device (2) to an injection device (1), wherein the injection device (1) has an elongated housing (10) extending in an axial direction (3, 4), the mounting aid (400) comprises:a body (401) extending in axial direction (3, 4), configured to receive at least a portion of the housing (10) and having at least one radially extending engaging member (406, 416) to engage with a correspondingly shaped engaging member (456) of the housing (10) when in a specific position (M) relative to the housing (10), andwherein the body (401) comprises a side wall (402) with an abutment face (410) facing in a proximal direction (4), wherein the abutment face (410) is defined on a plane that is inclined to the axial direction (3, 4).
