The previous answer by Gotlus is correct.
I just had exactly the same problem with my K12 Micra, however, when I dried the pump out out on the radiator overnight, it worked the next morning when I refitted it, but the problem returned later that day. In my case I think that the water must have been getting into the motor from the washer reservoir rather than an external source (e.g. rain).
I fitted a new pump and everything is now working correctly.
Dave
December 2012