Dynamic Dispatch, Kotlin

I have this code

open class Shape {    open fun number(m: Shape) = "1"    open fun number(m: Rectangle) = "2"}class Rectangle : Shape() {    override fun number(m: Shape) = "3"    override fun number(m: Rectangle) = "4"}fun main() {    val shape: Shape = Rectangle()    println(shape.number(shape))}

I expected 4 to be printed, while it is 3: how does dynamic dispatch work? the invoked function is Rectangle#number(Shape) even tho the object is the same, shape.