U.S. stock index futures added to losses on Wednesday after disappointing quarterly results from Morgan Stanley and Wells Fargo & Co .

Shares of both banks were down more than 5 percent in premarket trade.

S&P 500 futures fell 8.6 points and were below 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 lost 68 points and Nasdaq 100 futures fell 6.5 points.

(Reporting by Rodrigo Campos; Editing by Padraic Cassidy)