Smart cast to 'Type' is impossible, because 'variable' is a mutable property that could have been changed by this time

And the Kotlin newbie asks, "why won't the following code compile?":

var left: Node? = null
    
fun show() {
    if (left != null) {
        queue.add(left) // ERROR HERE
    }
}

Smart cast to 'Node' is impossible, because 'left' is a mutable property that could have been changed by this time I get that left is mutable variable, but I'm explicitly checking left != null and left is of type Node so why can't it be smart-casted to that type? How can I fix this elegantly?