Why does this return "You still owe: None"
<pre><code>def hotel_cost(nights):
    return nights * 140

bill = hotel_cost(5)

def add_monthly_interest(balance):
    balance * (1 + (0.15 / 12))

def make_payment(payment, balance): 
    new_balance = add_monthly_interest(balance)
    print "You still owe: " + str(new_balance)

make_payment(100,hotel_cost(5))
</code></pre>

<p>This is printing "You still owe: None" and I feel like I am just missing something very basic.  I am about as new as it could possibly get.  Python is my first language, no other real computer knowledge beyond being as technologically literate as most in my generation.</p>
