Disclosed herein is a tablet comprising crystalline (S)-isopropyl 2-(((S)-(((2R,3R,4R,5R)-5-(2,4-dioxo-3,4-dihydropyrimidin-1 (2H)-yl)-4-fluoro-3-hydroxy-4-methyltetrahydrofuran-2-yl)methoxy)(phenoxy)phosphoryl)amino)propanoate represented by the formula Sp-4:having XRPD 2-theta-reflections (±0.2°) at 6.1 and 12.7.This tablet is for use in the treatment of a hepatitis C virus infection.