body joint angles using kinect

Question

hi,i have  just  working on kinect and c# programming so i am very amateur at it
i liked to know  about joint angles

i created this code just to test me skill without  using kinect 
but em having trouble with this code 
i supposed   vector3 v1 and vector3 v2 as a joint replacement 
but the value return from  Find_angle   is 'nan'
am i missing something in the process
please any help in this regard will b helpful

Vector3 V1 = new Vector3(100,40,90);

 Vector3 v2 = new Vector3(160,60,90);

public MainWindow()
        {
            InitializeComponent();

            Vector3.Normalize(V1);
            Vector3.Normalize(v2);

            float Result = this.find_angle(V1,v2);
            MessageBox.Show(Result.ToString());

}

 public float find_angle(Vector3 va, Vector3 vb)
        {
            float dot_pro=Vector3.Dot(va, vb);
            double angle = Math.Acos(dot_pro);

            angle = angle * 180 / Math.PI;

            return (float)(angle);
        }