The invention relates to a negative-pressure device (1) for affixing a patient interface (2) with a negative-pressure cavity (20) onto a patient eye. The negative-pressure device (1) comprises a negative-pressure generator (10) and a pressure sensor (11) with a device-side pressure sensor interface (14) for coupling the pressure sensor (11) to the negative-pressure cavity in a manner fluidically separate from the negative-pressure interface (13). The negative-pressure device furthermore relates to a control unit (12) operatively coupled to the negative-pressure generator (10) and the pressure sensor (11), wherein the control unit (12) is designed to detect a faulty fluidic coupling of the negative-pressure cavity (20) by evaluating a pressure established by the pressure sensor (11). The invention furthermore relates to a patient interface (2) with a negative-pressure cavity (20), wherein the negative-pressure cavity (20) has a negative-pressure interface and a fluidically separate pressure sensor interface, as well as a patient interface with an integrated pressure sensor. The invention furthermore relates to a method for coupling a patient interface (2) to a patient eye.