Argparse error when inputting values

[tqader]

I'm trying to resolve the errors in this code. We are trying to use argparse, and I get to num_nums = len(args.n) and I get errors. The error gets generated when reading the next line and reads as:

ERROR CODE

Error:
print("Adding %d numbers." % num_nums)
TypeError: %d format: a number is required, not NoneType

I can't figure out what is failing with the %d argument that is being passed.
I am looking for assistance with the code to get it to a working state. Full code is:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("-n", help="Number to add")
parser.add_argument("--indent", help="Indent output")

args = parser.parse_args()

num_nums = (args.n)
print("Adding %d numbers." % num_nums)
sum = 0
for n in range(0, num_nums):
    sum = sum + args.n[n]

if (args.i):
    print("\tSum = %d" % sum)
else:
    print("Sum = %d" % sum)