What is the equilibrium constant of the reaction?

$$NO_{3}^{-} + NADH + H^{+} <=> NO_{2}^{-} + NAD+ + H_{2}O$$

I wrote in an exam that the [NADH] and [H2O] remain constant in the reaction.

$$K = \frac {[NO_{2}^{-}] [NAD^{+}]} {[NO_{3}^{-}] [H^{+}]}$$
which is wrong.

$$K = \frac {[NO_{2}^{-}] [NAD^{+}]} {[NO_{3}^{-}] [H^{+}] [NADH]}$$

So why is [NADH] not constant in the reaction?