A recently proposed model to describe the galvanostatic oscillating oxidation of hydrogen on Pt in the presence of copper and chloride is also able to explain the potentiostatic behaviour of the experimental system. For sufficiently high series resistance it exhibits current oscillations on a branch where the stationary current-voltage curve has a positive polarization slope. This is in contrast to most of the potentiostatic oscillators examined and described previously, where the oscillations are caused by an interplay between the IR drop and a region of negative polarization slope. The mechanism is discussed and compared with other models which describe potentiostatic oscillators. The main difference is shown to lie in the existence of a hidden negative impedance, which is detected experimentally by impedance spectroscopy.