A prophylactic device for protecting the diaphragm of a pump, such as a diaphragm pump for a breastpump, from fluid (air/milk) is disclosed. A removably mounted flexible cover is located between the shell of the pump and the movable diaphragm membrane within the shell which isolates the membrane from fluid. The cover is removable for at least one of cleaning and disposal. The flexible membrane has a circumferential rim upon which the cover is received over the rim. The shell has an internal opening defined therein sized to encompass the rim with the cover mounted on the rim in a substantially airtight fit. The cover thereby forms a gasket between the rim and shell.