The electrical and thermal Hall conductivities of the cuprate superconductor YBa2Cu3Oy, σxy and κxy, were measured in a magnetic field up to 35 T, at a hole concentration (doping) p=0.11. In the T=0 limit, we find that the Wiedemann-Franz law, κxy/T=(π2/3)(kB/e)2σxy, is satisfied for fields immediately above the vortex-melting field Hvs. This rules out the existence of a vortex liquid at T=0 and it puts a clear constraint on the nature of the normal state in underdoped cuprates, in a region of the doping phase diagram where charge-density-wave order is known to exist. As the temperature is raised, the Lorenz ratio, Lxy=κxy/(σxyT), decreases rapidly, indicating that strong small-q scattering processes are involved. © 2016 American Physical Society.