### How-to: sum

Suppose I have defined a function
f(x)=x^2 and a list
x=[0,1,2,3,4]

Now I can calculate the product x[2]*f(x[2]).
But: If I use the command

k=var('k')
sum(x[k]*f(x[k]),k,0,4)

I will get the message: "TypeError: unable to convert x (=k) to an integer"

How can I realize such a summation?