[nine_k]

It does if you specify a sequence of list as the first argument, and an empty list as an initial value.

[vostok]

This is because the start argument defaults to 0. It doesn't make sense to add a list and 0.

The same thing does not work for a sequence of strings and an empty string.

    sum(["abc","def"], "")

Throws TypeError: sum() can't sum strings [use ''.join(seq) instead]