Simply detecting the HTC Vive presence

I am using the following code but it does not work in the statement that checks if the hmd is disconnected

Code (CSharp):

1.      public bool isVREnable;
2.      private void Update()
3.      {
4.          if (XRDevice.isPresent)
5.          {
6.              isVREnable = true;
7.              Debug.Log("Vive is connected");
8.  
9.          }
10.          else
11.          {
12.              isVREnable = false;
13.              Debug.Log("Vive is not connected");
14.          }
15.      }