title: cv2.videocapture.read() does not return a numpy array
body: <p>I have this code trying to capture a frame from my webcam on raspberry pi, and saving it as an image. I use opencv 2, but I get strange errors when I run the code..</p>
  
  <pre><code>import time
  import sys
  from subprocess import call
  import cv2
  
  cam = cv2.VideoCapture()
  
  while True:
          cam.open(-1)
          image = cam.read()
          cv2.imwrite("current.jpeg",image)
          time.sleep(10);
  </code></pre>
  
  <p>This is what the program returns: </p>
  
  <pre><code>VIDIOC_QUERYMENU: Invalid argument
  VIDIOC_QUERYMENU: Invalid argument
  VIDIOC_QUERYMENU: Invalid argument
  VIDIOC_QUERYMENU: Invalid argument
  VIDIOC_QUERYMENU: Invalid argument
  VIDIOC_QUERYMENU: Invalid argument
  VIDIOC_QUERYMENU: Invalid argument
  
  Traceback (most recent call last):
    File "kvamskogen.py", line 18, in &lt;module&gt;
      cv2.imwrite("current.jpeg",image)
  TypeError: &lt;unknown&gt; is not a numpy array
  </code></pre>
  
  <p>What is wrong here?</p>