How can I fix this type error in recursive program?

''' Write a function that takes an integer n and returns the nth iteration of the fractal known as Sierpinski's Gasket. Here are the first few iterations. The fractal is composed entirely of L and white-space characters; each character has one space between it and the next (or a newline).

0
L

1
L
L L

2
L
L L
L   L
L L L L

3
L
L L
L   L
L L L L
L       L
L L     L L
L   L   L   L
L L L L L L L L


def sierpinski(n):
    if n==0:
        return "L"
    print(sierpinski(n-1)+"\n"+sierpinski(n-1)+" "+sierpinski(n-1))


sierpinski(1)
sierpinski(2)

/Library/Frameworks/Python.framework/Versions/3.4/bin/python3.4 /Users/mona/PycharmProjects/PythonCodes/sierpinski.py L Traceback (most recent call last): L L File "/Users/mona/PycharmProjects/PythonCodes/sierpinski.py", line 38, in <module> L sierpinski(2) L L File "/Users/mona/PycharmProjects/PythonCodes/sierpinski.py", line 34, in sierpinski print(sierpinski(n-1)+"\n"+sierpinski(n-1)+" "+sierpinski(n-1)) TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Process finished with exit code 1