LCOGT MuSCAT3 light curves of K2-399. The light curves from top to bottom are in the MuSCAT g, r, i, and z_s bands. The small symbols show the unbinned data and the larger symbols show the same data in ten-minute bins. The transit model fits are overplotted. A V-shaped event was detected on-target with depths at mid-transit of 0.18, 0.48, 1.21, and 2.28 ppt in g, r, i, and z_s bands, respectively, indicating that a fainter, much redder star is blended in the 5.1 arcsec follow-up photometric aperture and is hosting the eclipse.