sum works in sequences (Python 3)

[Franck Ditter]

Hello,
I wonder why sum does not work on the string sequence in Python 3 :

>>> sum((8,5,9,3))
25
>>> sum([5,8,3,9,2])
27
>>> sum('rtarze')
TypeError: unsupported operand type(s) for +: 'int' and 'str'

I naively thought that sum('abc') would expand to 'a'+'b'+'c' 
And the error message is somewhat cryptic...

    franck