S&P 500 stock index futures fell and Dow futures turned negative on Wednesday after package delivery company FedEx Corp forecast an extremely difficult operating environment for the first two quarters of its fiscal year.

The company is seen as a window on the economy, servicing many varied sectors.

S&P 500 futures fell 3.10 points and were roughly in line with fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 19 points, while Nasdaq 100 futures added 0.25 points.

(Reporting by Edward Krudy; Editing by Padraic Cassidy)