Cannot get correct image from camera sensors when camera look straight down

Hi team,
I cannot get correct image from camera sensors when camera look straight down. I set camera_location = (0.35, 0.65, 0.75) and camera_lookat =(0.35, 0.65, 0), and I got an all-black picture.
However, when I set camera_location = (0.35, 0.65, 0.75) and camera_lookat =(0.35, 0.651, 0), with just slightly offset, I got the correct image.
What is this about?