coroutine bug

extends Node2D

# member variables here, example:
# var a=2
# var b="textvar"
func coroutine():
    print("2-1")
    yield()
    print("2-2")
    yield()
    print("2-3")
    yield()
    print("2-4")

func _ready():
    print("Starting")
    var co = coroutine()
    while co.is_valid():
        co.resume()
    print("Complete")