Odd return value compared to print value

Question

I am writing a function to make a 'digital root', which adds all digits of a number together recursively until it is one digit.

When I ask the function to return a value, it returns 'None', but if I ask it to print the value, it returns the correct answer.

Code:

def digital_root(n):
    if len(str(n)) != 1:
        list = []
        for i in str(n):
            list.append(int(i))
        n = sum(list)
        digital_root(n)
    else:
        return n