Classical entropy as phase space volume

elemental09

The entropy of a classical system of N particles is normally (see e.g. Huang p.131) defined as the k_b times the natural log of the volume of phase space accessible to the system.
My question is: how can we take the logarithm of the phase space volume, when the volume has units? For N particles, the phase space volume has units [mass]^3N*[distance]^6N*[time]^(-3N).