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Linear stability of Israel-Stewart theory in the presence of net-charge diffusion was investigated in [C. V. Brito and G. S. Denicol, Linear stability of Israel-Stewart theory in the presence of net-charge diffusion, Phys. Rev. D 102, 116009 (2020)] for the case of a massless, classical gas of noninteracting particles. However, in that work only a vanishing net-charge background was considered. In this work, we extend that study to the case of a nonvanishing background charge. We find that this effectively results in a change of the numeric value of the charge-diffusion coefficient, in a way that when the background charge goes to infinity, this coefficient can become at most four times its value at zero background charge. We also extend the analysis of Brito and Denicol [Linear stability of Israel-Stewart theory in the presence of net-charge diffusion, Phys. Rev. D 102, 116009 (2020)] by performing a systematic parameter study in the plane of charge-diffusion coefficient vs the coupling term between shear-stress and net-charge diffusion. In this plane, we identify regions where the solutions remain stable and causal and where they become acausal and/or unstable.