specifying user input in delimiter in csv reader

Question

the following code works,

reader = csv.reader(csvfile, delimiter='\t')

but if I specify,

args_parser = argparse.ArgumentParser()
args_parser.add_argument('delimiter', nargs='?', default='')
args = args_parser.parse_args()

give \t as input, and specify,

reader = csv.reader(csvfile, delimiter=args.delimiter)

then it gives error,

TypeError: "delimiter" must be a 1-character string

How do I pass user input to the delimiter argument of csv.reader