sum of datetime.datetime object gave an error TypeError: unsupported operand type(s) for +: 'datetime.datetime' and 'datetime.datetime'

sum of datetime.datetime object gave an error TypeError: unsupported
operand type(s) for +: 'datetime.datetime' and 'datetime.datetime'

I'm trying to sum a list of datetime.datetime object using :
from datetime import datetime, timedelta
d= [datetime.datetime(2013, 5, 1, 9, 31, 24), datetime.datetime(2013, 6,
11, 17, 22, 18), datetime.datetime(2013, 4, 3, 16, 6, 59)]
sum_d = sum(d, timedelta())
I get the error :
TypeError: unsupported operand type(s) for +: 'datetime.datetime' and
'datetime.datetime'
any idea how to sum up this list?
Thanks!