debug screen reports light level as (0 sky, 0 block)

I am trying to see why my villagers keep dying, so I wander around the village looking for spots with light less than 8. After less than 2 minutes of doing this (fairly consistently) the debug screen stops reporting the light level.

Let me know if there is some way I can more completely describe the bug. Any tools to get debug logging or such? I am a java developer but I do not decompile.