A breast pump device (1) comprises an expression kit (2), a vacuum unit (3), and a conduit (5) for establishing an air path between an air outlet (27) of the expression kit (2) and an air inlet (35) of the vacuum unit (3). The expression kit (2) is equipped with a barrier portion (26) for preventing milk leakage from the expression kit (2) towards the hose (5) and the vacuum unit (3). In case some droplets of milk enter the barrier portion (26) and/or drawn through the barrier portion (26) by an ongoing pumping action, aspects of the behavior of the motor (32) for driving the pump (31) are influenced, and the insight that this happens is used for terminating operation of the motor (32) as soon as a too large deviation of one or more operational characteristics of the motor (32) with respect to a reference is found.