  We present a 77Se NMR study of the newly discovered iron selenide
superconductor KyFe2-xSe2, in which Tc = 32 K. Below Tc, the Knight shift 77K
drops sharply with temperature, providing strong evidence for singlet pairing.
Above Tc, Korringa-type relaxation indicates Fermi-liquid behavior. Our
experimental results set strict constraints on the nature of possible theories
for the mechanism of high-Tc superconductivity in this iron selenide system.
