A reverse osmosis system that includes a housing having an inlet port, a permeate port and a concentrate port. A membrane element is within the housing. The reverse osmosis system further includes (i) a first connector that is connected to the inlet port; (ii) a second connector that is connected to the permeate port; and (iii) a third connector that is connected to the concentrate port. A key is inserted between each of the first, second and third connectors. The key is configured such that the key cannot be inserted between the first, second and third connectors unless each of the first, second and third connectors is properly connected to the respective inlet, permeate and concentrate ports. In some embodiments, the key is configured such that the key cannot be removed from between each of the first, second and third connectors when there is pressure at the inlet port.