The present invention relates to a cap comprising a body, an opening passing through the cap and adapted in turn to be passed through by at least one product transfer member, and a membrane which, at rest, covers the opening. The membrane has a main portion that extends through the opening and defines two inclined faces, each inclined face having a distal edge. The two inclined faces form a dihedron when the membrane is at rest, the distal edges of the two inclined faces coming together at the apex of the dihedron. The cap comprises at least two flaps that extend through the opening, above the membrane, the two inclined faces of the membrane being respectively covered by two flaps, each flap having a free edge that extends along the distal edge of the corresponding inclined face.