  Crystallographic defects in a topological semimetal can result in charge
doping, and the scattering due to the defects may mask its exotic transport
properties. Here, we investigate the possible crystallographic defects
including vacancy and antisite in Weyl semimetal LaAlGe using
hybrid-density-functional theory calculations. We show that a considerable
concentration of Al- and Ge-related defects naturally form during growth due to
their low formation enthalpy. Specifically, Al can be easily replaced by Ge in
the $I4_1md$ phase of LaAlGe, forming the Ge-on-Al antisite, Ge$_{\rm Al}$. The
counterpart, Al-on-Ge (Al$_{\rm Ge}$), is also probable. The most abundant
defect Ge$_{\rm Al}$ is donor-like, effectively electron-doping, and these
defects are therefore not only scattering centers in the electronic transport
process but may also induce the substantial vertical shift of the chemical
potential. The results imply that the naturally occurring defects hinder both
spectroscopic and transport features arising from the Weyl physics in LaAlGe.
Our work can be applied to the $R$AlGe family ($R$=rare earth) and help improve
the quality of single-crystal magnetic Weyl semimetal.
