The released version 2.1k has been found to have a bug in the Voigt profile calculation of absorption line profiles. This causes Infs in the opacity for lines with small damping parameters on some machines. Xstar itself does not halt due to this error, and produces optical depths which are large in the core of the affected lines. Xstar2xspec can halt with an arithmetic error caused by the inability of fitsio to read the large opacity. This bug has been repaired in version 2.1kn and later.