AttributeError: '_Screen' object has no attribute 'mainloop'

I'm designing a simple Python program that uses the Turtle graphics module to draw lines on the screen with the arrow keys.

import turtle

turtle.setup(400,500)                # Determine the window size
wn = turtle.Screen()                 # Get a reference to the window
wn.title("Handling keypresses!")     # Change the window title
wn.bgcolor("lightgreen")             # Set the background color
tess = turtle.Turtle()               # Create our favorite turtle

# The next four functions are our "event handlers".
def h1():
   tess.forward(30)

def h2():
   tess.left(45)

def h3():
   tess.right(45)

def h4():
    wn.bye()                        # Close down the turtle window

# These lines "wire up" keypresses to the handlers we've defined.
wn.onkey(h1, "Up")
wn.onkey(h2, "Left")
wn.onkey(h3, "Right")
wn.onkey(h4, "q")

# Now we need to tell the window to start listening for events,
# If any of the keys that we're monitoring is pressed, its
# handler will be called.
wn.listen()
wn.mainloop()

When I try to execute it, the following error is returned.

Traceback (most recent call last):
  File "C:\Users\Noah Huber-Feely\Desktop\PEN_Programming\Python\etchy_sketch1.py", line 32, in <module>
    wn.mainloop()
AttributeError: '_Screen' object has no attribute 'mainloop'

I'm using Python 2.7 and have had no trouble with Turtle graphics before. It is only now that I have begun working with key input that this problem has occurred.

Searching on the web, I only found articles about different issues and modules than the one I am currently experiencing.

Let me know if you need more information. Thanks!