Chances are, the nitrate reading you're getting, it's very low anyway, is probably from your tap water.
From my own tap water I get between 10 to 20 ppm nitrates.
Also don't forget, nitrate test can be a little unreliable to the reagent solution needing to be shaken like there's no tomorrow!
At this stage, you're still in beginning of Fishless cycle stages.
Good luck with that and hope its a fast cycle, especially if you're using media from one of your established tanks if you've any.