We report As-75 and F-19 NMR studies on the superconducting properties of the newly discovered compound PrFeAsO0.89F0.11 (T-c = 45 K). The Knight shift in the direction of H parallel to ab-plane decreases below T-c to almost zero, which indicates spin-singlet pairing. The temperature dependencies of both the Knight shift and the spin lattice relaxation rate indicate that there are two gaps opening below T-c, with nodes in the gap function. Copyright (c) EPLA, 2008.