Boat error.

Ok so me and my friend play on a private server that I host. I made a boat and paced it into the water(trying to get across the ocean)but then when I got on in the boat I got an error. I believe it said java pointer exception null. I use hamachi and to reproduce this you just need to place a boat on a vanilla minecraft server and try to get into it, which should then give you the error. Hope this helps