C:\Dev-Cpp\lib\vector.h `ostream' is neither function nor member function; cannot be declared friend

#ifndef VECTOR_H_
#define VECTOR_H_
#include <iostream>
class Vector
{
    private:
        double x;
        double y;
        double mag;
        double ang;
        char mode;
        
        void set_mag();
        void set_ang();
        void set_x();
        void set_y();
    public:
        Vector(double n1, double n2, char form = 'r');
        void set(double n1, double n2, char form = 'r');
        double xval() const {return x;};
        double yval() const {return y;};
        double magval() const {return mag;};
        double angval() const {return ang;};
        void polar_mode() {mode='p';}
        void rect_mode() {mode='r';}
        Vector operator*(double n);
        friend Vector operator+(Vector & v1, Vector & v2);
        friend Vector operator-(Vector & v1, Vector & v2);
        friend Vector operator*(double n, Vector & v);
        Vector operator~();
        friend ostream & operator<<(ostream & os, Vector & v);
};
#endif

#include "vector.h"

friend std::ostream & operator<<(std::ostream & os, Vector & v);