Question

The kinetic parameters of the reaction of lactate to pyruvate can be measured by following the formation of NADH from NAD+. It can also be measured in the reverse direction, however, at high concentrations of NADH the reaction cannot be monitored at 340 nm? Why is this?

I'm a bit confused at why high NADH concentrations would make it impossible to monitor a reaction at 340 nm. Any explanation of this concept would be greatly appreciated.