We investigate the effects of carbon and boron doping on the thermal expansion of the hexagonal $(P{6}_3∕mmc)$ intermetallic YbGaGe. X-ray powder diffraction was used to measure the lattice constants on pure and doped (C or B at nominal levels of 0.5%) samples from $T\sim 10K\text{to}T\sim 300K$. Also measured were resistivity, specific heat, and magnetic susceptibility. While the pure YbGaGe samples exhibit positive thermal volume expansion, $\left(V_{300K}\text{−}{V}_{10K}\right)/V_{300K}=0.94$, the volume expansion in the lightly C- or B-doped samples changes slope and tends toward zero-volume expansion. Such a strong response to light doping suggests that the underlying mechanism for the reported zero-volume expansion is substitutional disorder, and not the previously proposed valence fluctuations.