valarray divide it's element

when I divide a valarray by it's first element, only the first element becomes 1 and others keep their origin value.

#include <iostream>
#include <valarray>
using namespace std;

int main() {
    valarray<double> arr({5,10,15,20,25});
    arr=arr/arr[0]; // or arr/=arr[0];

    for(int value:arr)cout << value << ' ';
    return 0;
}

output is

1 10 15 20 25

why the output is not

1 2 3 4 5

I use g++(4.8.1) with -std=c++11