  In a recent paper, Iba\~nez-Azpiroz et al. [Phys. Rev. B 97, 245143 (2018)]
derive a band-truncation-error-free formula for calculating the generalized
derivative of the interband dipole matrix using Wannier interpolation. In
practice, the denominators involving intermediate states are regularized by
introducing a finite broadening parameter. In this Comment, I show that when a
finite broadening parameter is used, a correction term must be added to the
generalized derivative to obtain results that are independent of the phase
convention for the Bloch sums.
