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Hey dear
Try storing the string into a variable first and then try to add them...
This might help you
Thanks ;)

But if it still not works then I think that the sum () function can only add integers and floats

It's because result is set to 0 here: https://github.com/python/cpython/blob/985f0ab3ad5e8e9a8d7fc53026c38390b1f2b466/Python/bltinmodule.c#L2296

Then Python basically tries to do:

result = 0
for item in ('a', 'b', 'c'):
    result += item

You can create a bug report at bugs.python.org and ask if it's possible to improve error message, but I'd expect it's going to be rejected since it's a rare usage.