I'm doing the digit_sum exercise on the Practice makes perfect section of the Python course in Codecademy

I'm getting the message:

Oops, try again. Your function fails on digit_sum(434). It returns 4 when it should return 11.

def digit_sum(n):

added_number = 0

for i in str(n):

str(i).split()

added_number += int(i)

return added_number

number = raw_input("Enter your chosen number here: ")

digit_sum(number)

When I run the code, it prompts for the user input. I enter: 1234

This is the output:

Enter your chosen number here: 1234

None

I have no idea what I'm doing wrong. I don't see where 4 is returned. Please help.