In Robinet et al. (2000), an instability for shock waves in gas dynamics is exhibited. The fluid was assumed to obey the polytropic perfect gas pressure law. We show in this note that such an instability does not occur, as proved in the extensive work of Majda (1983). Two arguments are developed: we give first a mathematical result that is violated by the conclusions of Robinet et al. (2000). Then we detail the results of Robinet et al. (2000) and show why they do not yield any conclusions. Finally, we develop a general calculation and show that an instability cannot occur.