WebCamTexture GetPixel(s) not working (RGB values are 0)

I am not able to get valid RGB values from the camera using WebCamTexture.

Playing the live stream of the camera using a RawImage just works fine.

But if I try to get the pixels out of a frame I always get RGB(0,0,0):

 cam  = new WebCamTexture(devices[0].name, 640, 480, 25);

 rawImageSrc.texture = cam; // works fine: Image in GUI will render the live stream
 rawImageSrc.material.mainTexture = cam; 
 cam.Play();

 if (cam.isPlaying)
 {
   Color col = cam.GetPixel(0, 0);       // rgb: 0,0,0
   Color[] cols = cam.GetPixels();       // rgb: 0,0,0
   Color32[] cols32 = cam.GetPixels32(); // rgb: 0,0,0
 }

I found several posts where people have the same issue. Unfortunately, there are no solutions about how to fix the issue in the posts.

Anybody an idea what I am doing wrong?

Or is there a way to get the pixel data from RawImage?

I am using Unity 5.2.1f1 Personal.

Thanks in advance.