why is acc here unbound

Hello,

I have this :

let length list =
     let rec length2 list2 acc =
         match list2 with
            | [] -> acc
            | x::xs -> length2 xs (acc + 1)
     in acc 0 list2 list ;;

But as soon as I copie it into the ocaml prompt I see a message that the 
acc in the part in acc 0 list2 list is unbound.

Roelof