问Python3 中Collatz sequence的问题？

def collatz(n):

if n % 2 == 0:

n=n//2

else:

n=n*3 + 1

print('Please type a number')

m=int(input())

while m != 1:

m=collatz(m)

print(m)

总是提示我

Traceback (most recent call last):

File "D:/Python/Collatz sequence.py", line 10, in <module>

m=collatz(m)

File "D:/Python/Collatz sequence.py", line 3, in collatz

if n % 2 == 0:

TypeError: unsupported operand type(s) for %: 'NoneType' and 'int'