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

[Jordan Harry]

I'm trying to write a simple program to calculate permutations.  I created a file called "mod.py" and put the following in it:
 
def factorial(n):
    a = n
    b = n
    while a>0 and b>1:
        n = (n)*(b-1)
        b = b-1
 
def perm(n, r):
    a = factorial(n)
    b = factorial(n-r)
    q = a / b
    print q

Then I went back to IDLE and input the following:
 
>>> import mod
>>> mod.perm(5, 4)
 
I recieved the following error message:
 
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    mod.perm(5, 4)
  File "C:\Python25\mod.py", line 27, in perm
    q = a / b
TypeError: unsupported operand type(s) for /: 'NoneType' and 'NoneType'
 
I have no idea how to fix it.  I'm pretty new to Python.  I have IDLE 1.2.2 and Python 2.5.2,
 
Please help.