C++ :: Vector Out Of Range Exception

I'm trying to do some operator overloading, the function is supposed to add the values at index [i] of two vectors and place the result in the returning vector. The problem is I keep getting a vector out of range. This is the overloaded operator I'm working with (relatively new to these):

vector<float> operator+(const vector<float>& a, const vector<float>& b){
unsigned long size;
vector<float> temp;
if(a.size() >= b.size())
size = a.size();

[Code] .....

and then I would do something like this in the main:

vector<float> v, v1, v2;

v1.push_back(9.1);
...
v2.push_back(8);
...
v = v1 + v2;

but when I try to output the vector v I just get a vector out of range exception.