Root Vehicle Tag does not work

I was working with the scoreboard when I learned of an easier way, so I tried to use it but noticed that this doesn't work:

testfor @a {RootVehicle:{Entity:{id:"Boat"}}}

I actually was using:

testfor @a {RootVehicle:{Entity:{id:"EntityHorse"}}}
testfor @a {RootVehicle:{Entity:{id:"Horse"}}}

trying to get it to work and it doesn't even work when I am in a boat.

If i am doing it wrong my apologies. It's hard to find this tag's information,