A Royal Navy warship has returned to port in Britain after several sailors got sick from the vessel’s drinking water, officials said Saturday. 

The navy said frigate HMS Portland had returned to its base at Portsmouth on England’s south coast on Friday “as a precautionary measure, following an issue with one of the ship’s fresh water systems.”

It said “a small number of personnel were taken to hospital as a precaution.”

The Daily Telegraph reported that a crew member put the wrong chemicals into the system that converts sea water to drinking water, but quickly realized the mistake and reported it to their superiors.

In recent months HMS Portland, a Type 23 frigate, has tracked Russian submarines in the North Sea, and helped monitor a Russian frigate and accompanying tanker as they sailed in international waters near the U.K.