  This comment points out errors in the analysis of 61 magnetic hyperfine
structure ($A$) constants of Co II energy levels by Fu et al. [JQSRT 2021,
107590]. The paper was published without full awareness of the extensive
literature already available for Co II hyperfine $A$ constants at the time; 57
of 58 $A$ constants that were claimed to have been measured for the first time
had already been measured by the prior work of Ding \& Pickering [ApJS 2020,
251:24], who had published $A$ constants for 292 levels of Co II. The $A$
constant of 3d$^6$4s$^2$ a$^5$D$_4$ has been determined by Fu et al. [JQSRT
2021, 107590] for the first time to be $12.0\pm1.8$ mK (1 mK $=$ 0.001
cm$^{-1}$), which was found to agree with line profiles observed by Ding \&
Pickering [ApJS 2020, 251:24]. Discrepancies in 17 $A$ constants of Fu et al.
[JQSRT 2021, 107590] were found, which are likely due to the analysis of weak,
experimentally unclassified transitions with Ritz wavenumbers 25453.966
cm$^{-1}$ and 25149.948 cm$^{-1}$ by Fu et al. [JQSRT 2021, 107590] for the $A$
constants of the energy levels 3d$^7$($^2$G)4s a$^3$G$_5$ and 3d$^7$($^2$P)4s
c$^3$P$_2$ respectively. Fewer transitions and poorer quality spectra analysed
by Fu et al. [JQSRT 2021, 107590] are also concluded to have contributed to
disagreements in the 17 $A$ constants.
