Why does OpenCV think I am using a nonplanar calibration setup?

I am playing with a simulated camera to try and understand how the OpenCV calib3d module works and works.

I created an artificial set of object points in 3D space corresponding to a flat grid of nine points at z = 50:

obj_pts = np.zeros((9, 3), dtype='float32')
obj_pts[0] = np.array([40, 40, 50], dtype='float32')
obj_pts[1] = np.array([50, 40, 50], dtype='float32')
obj_pts[2] = np.array([60, 40, 50], dtype='float32')
obj_pts[3] = np.array([40, 50, 50], dtype='float32')
obj_pts[4] = np.array([50, 50, 50], dtype='float32')
obj_pts[5] = np.array([60, 50, 50], dtype='float32')
obj_pts[6] = np.array([40, 60, 50], dtype='float32')
obj_pts[7] = np.array([50, 60, 50], dtype='float32')
obj_pts[8] = np.array([60, 60, 50], dtype='float32')

      

        
        
        
      

    

What I then depicted with the help cv2.projectPoints()

after creating the artificial camera:

rvec = (0, 0, 0)  # rotation relative to the frame
tvec = (0, 0, 0)  # translation relative to the frame
distCoeffs = (0, 0, 0, 0)
cameraMatrix = np.zeros((3, 3))
focalLength = 50
cx = 0
cy = 0
setupCameraMatrix(cameraMatrix, focalLength, cx, cy) # my own routine

img_pts, jacobian = cv2.projectPoints(obj_pts, rvec, tvec, cameraMatrix, distCoeffs)

      

        
        
        
      

    

Projected onto the image plane using the above parameters, the image points look like this (the red dot only indicates the lower left corner for orientation):

And finally, I'm trying to restore the original camera calibration:

obj_pts_list = [obj_pts]
img_pts_list = [img_pts]
ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(obj_pts_list, img_pts_list, (200, 200), None, None)

      

        
        
        
      

    

However, this last step gives this error:

OpenCV Error: Bad argument (For non-planar calibration rigs the initial intrinsic matrix must be specified) in cvCalibrateCamera2, file /tmp/opencv20150527-4924-hjrvz/opencv-2.4.11/modules/calib3d/src/calibration.cpp, line 1592

      

        
        
        
      

    

My question is not how to fix this error as such, but why is it being thrown in the first place? Why is this setup a nonplanar setup when all points of the object lie in the same plane? Did I get it wrong?