TypeError: 'NoneType' object is not iterable. Why do I get this error?

The function compute_root uses Newton's method of successive approximation to find good enough approximations of the zeroes of polynomials (herein lies the problem). The function evalPoly computes the value of a polynomial at a particular x value, and the function ddx2 computes the derivative of a polynomial.

poly2 = (2,3,1,5) #poly2 represents the polynomial 5x^3+x^2+3x+1

def evalPoly(poly,x):
    degree = 0
    ans = 0
    for index in poly:
        ans += (index * (x**degree))
        degree += 1
    return ans  

def ddx2(tpl):
    lst = list(tpl)
    for i in range(len(lst)):
        lst[i] = lst[i]*i
        if i != 0:
            lst[i-1] = lst[i]
    del lst[-1]
    tpl = tuple(lst)

def compute_root(poly,x_0):
    epsilon = .001
    numGuesses = 1
    if abs(evalPoly(poly,x_0)) <= epsilon:
        ans = (evalPoly(poly,x_0),numGuesses)
        print ans
        return ans
    else:
        x_1 = x_0 - (evalPoly(poly,x_0)/evalPoly(ddx2(poly),x_0))
        # This is Newton's method of getting progressively better / 
        # "guesses"
        compute_root(poly,x_1)
        x_0 = x_1
        numGuesses += 1
        return x_0
        return poly

compute_root(poly2,2) #Here I call the function *compute_root*

When I call the function I get this error:

Samuels-MacBook:python barnicle$ python problemset2.py
Traceback (most recent call last):
  File "problemset2.py", line 160, in <module>
    compute_root(poly2,x_0)
  File "problemset2.py", line 156, in compute_root
    x_1 = x_0 - (evalPoly(poly,x_0)/evalPoly(ddx2(poly),x_0))
  File "problemset2.py", line 126, in evalPoly
    for index in poly:
TypeError: 'NoneType' object is not iterable

I know python functions return none by default. I think that the error is produced because the value none is being passed into the parameter poly in evalPoly. Why is this happening?

I felt it would be prudent to include everything, even the function ddx2 (which hasn't been called yet in this example) because I don't know if you need it. I know compute_root needs a lot of work, this is just the first step. Thanks!!!

UPDATE!!!

It has been brought to my attention that I was getting the error because my function ddx2 lacked a return value, so it was of course returning the value none, which is of course not iterable. Thank you!!

UPDATE2!!!

I have my complete working program here which I am posting in the hope that it may help someone sometime. I spent a lot of hours on this. It's from MIT open courseware's electrical engineering and computer science 6.00sc with Professor John Guttag, problem set 2.

poly5 = (-13.39, 0.0, 17.5, 3.0, 1.0) # represents the polynomial:
       # x^4+3x^3+17.5x^2-13.39 


def evalPoly(poly,x):
    degree = 0
    ans = 0
    for index in poly:
        ans += (index * (x**degree))
        degree += 1 
    return float(ans)
    return degree   

def ddx2(tpl):
    lst = list(tpl)
    for i in range(len(lst)):
        lst[i] = lst[i]*i
        if i != 0:
            lst[i-1] = lst[i]
    del lst[-1]
    tpl = tuple(lst)
    return tpl

def compute_root(poly,x_0,numGuesses):
    epsilon = .001
    ans = []
    if abs(evalPoly(poly,x_0)) <= epsilon:
        ans.append(x_0)
        ans.append(numGuesses) 
        ans = tuple(ans)
        print ans
        return ans
    else:
        numGuesses += 1
        x_0 = x_0 - (evalPoly(poly,x_0)/evalPoly(ddx2(poly),x_0))
        compute_root(poly,x_0,numGuesses)

    compute_root(poly5,.1,1)

Output: Samuels-MacBook:python barnicle$ python problemset2.py (0.806790753796352, 8)

This program (is this a program?) only finds one real root, if one exists, but I suppose it is sufficient for an exercise.