F # generic constructor

Why does the following code give me the error "This code is not general enough"?

type Vector2D<'a when 'a : (static member (+) : 'a * 'a -> 'a)  > =
   struct 
      val X: 'a
      val Y: 'a
      new(x: 'a, y: 'a) = {
        X = x; 
        Y = y
        }
   end