The invention relates to a blood treatment cassette (1000) comprising a cassette body designed as a hard part (1) and comprising a film (3) wherein the film (3) is connected to the hard part (1) and covers the hard part (1) at least in some sections wherein the hard part (1) has at least one valve seat or valve section or a valve base (226) for a valve (288) wherein the valve (288) is designed to assume in addition to a first open position of the valve (288) in which the valve base (226) and a section of the film (3) arranged above the valve base do not touch each other a second closed position of the valve (288) when a force is applied to a section of the film (3) in which second position of the valve the valve base (226) and the section of the film (3) touch each other and wherein the valve base (226) has or consists of a non straight section in the longitudinal extent of the valve base. The invention further relates to a blood treatment device connected to a blood treatment cassette.
