Water leak when water resevoir is inserted TAS 1407CH/01 Type: CVTPM07?

When I insert my full water holder into the side of my Tassimo machine the water runs out from the junction of the machine and the water tank.
The water tank does not leak. Leakage only occurs when mating it with the machine.
It would appear there is some sort of sealing ring missing from the protrusion at the bottom of the water tank as there is a ring inside the the mating union of the machine.
If this is correct where can I obtain the missing ring please?
If I am wrong please put me right. Thank you very much.