Can't figure out what is wrong with my answer to the "combo" problem...

Hello ladies and gentlemen,

I feel pretty silly for asking this, but I simply cannot see what is wrong with my answer to the "combo" problem at the end of one of the Python Collections modules. Here is Kenneth Love's description of the problem:

Create a function named combo that takes two ordered iterables. These could be tuples, lists, strings, whatever. Your function should return a list of tuples. Each tuple should hold the first item in each iterable, then the second set, then the third, and so on. Assume the iterables will be the same length.

My code should be attached. I have run it on a number of test cases and they all came out correct, and I am not getting any useful output from the Treehouse answer checker. Thanks in advance!

# combo([1, 2, 3], 'abc')
# Output:
# [(1, 'a'), (2, 'b'), (3, 'c')]

def combo(iter_a, iter_b):
    length = len(iter_a)
    combo_list = []
    for i in range(0, length):
        combo_list.append((iter_a[i], iter_b[i]))
    return combo_list