On the issue 11, pp 2503-2514 of Computer Physics Communications, X. Xu et al published their research on numerical instabiliy due to relativistic plasma drift in EM-PIC simulations. In this paper, we show that a numerical instability arises from the intersection of beam resonances and electromagnetic modes in the drifting plasma. Spectral filters were applied in order to suppress this instability, which is critical to model extreme astrophysical scenarios such as the onset of relativistic shocks and plasma based accelerators in boosted frames. More information and the full text of the paper can be found .