What test kit are you using?
If it's the API liquid test kit then did you make sure you shook both nitrate test bottles vigorously before using. And did you add solution #1 shake + wait 30secs before adding solution #2.
If you did do all of the above then I haven't got a clue. Either ways I'd re-test if you haven't already(following instructions exactly and shaking all test solutions bottles just to be sure). It wont hurt to re-test, and it'd be very odd if you actually have 0 nitrates in the tank. Do you have a large amount of plants compared to fish?