An apparatus for selectably sealing an interior bore of a completion string comprises an elongate body and has connectors for connecting the body in line with a completion string. An interior wall of the apparatus forms a bore through the elongate body. A rupturable barrier member fastened to and bridging the bore is deformable so as to permit passage therepast after being ruptured. The barrier member includes at a weakened path defining a leaf extending from a hinge portion narrower than the widest portion of the leaf.