turtle.screen freezes and crash everytime i run my script : im making a space invader copy just to learn a little python since i just started with it and i made a turtle.screen but everytime i run it, it instantly freeze and crash.. does anyone know what's causing this problem?

   

> import turtle
> from turtle import forward, right, left
> forward(50)
> import os
> import math
> import random
> import shelve

> wn = turtle.Screen()
> wn.bgcolor("black")
> wn.title("Space invaders")

> border_pen = turtle.Turtle()
> border_pen.speed(0)
> border_pen.color("white")
> border_pen.penup()
> border_pen.setposition(-300, -300)
> border_pen.pendown()
> border_pen.pensize(3)
> for side in range(4):
>        border_pen.fd(600)
>        border_pen.lt(90)
> border_pen.hideturtle()

> delay = input("press enter to finish.")