Returning char from a string by index

I am using a random number generator to choose a number between 0 and the length of the string minus one. Then I am supposed to return the character in that index, but my code seems to have an error. Does anyone know what it is?

# EXAMPLE
# random_item("Treehouse")
# The randomly selected number is 4.
# The return value would be "h"
import random

def random_item(word = "Treehouse"):
num = random.randint(0,len(word)-1)
return word[num]