Minecart Motion Data Tag

Whenever an entity is riding a minecart or a boat with a motion value (such as this: /entitydata @e[type=MinecartRideable] {Motion:[1.0,0.0,0.0]} ), it goes through the ground. It still moves when something isn't in it, and it works fine, but breaks when I get in it. I would like this to get fixed soon, or tell me my problem.