Help on adding two lists recursively

[gontajones]

def recursive_sum(l1, l2, idx=0, carryout=0):
    if idx < min(len(l1), len(l2)):
        l1 = l1[::-1]
        l2 = l2[::-1]
        n = int(l1[idx]) + int(l2[idx]) + carryout
        carryout = 0
        if n > 999:
            carryout = int(n/1000)
            n -= 1000*carryout
        return recursive_sum(l1, l2, idx + 1, carryout) + [n]
    else:
        return [carryout] if carryout else []


l1 = ['001', '234', '567']
l2 = ['007', '894', '561']
# l2 = ['999', '166', '568']
print(recursive_sum(l1, l2))