The measuring system for in vivo monitoring of an analyte concentration with malfunction detection, comprises an electrode system, a potentiostat and an evaluation unit. The electrode system has a working electrode, a reference electrode, and a counter electrode. The potentiostat is for adjusting a difference of potential between the electric potential of the working electrode and the electric potential of the reference electrode to a specified value and for measuring an electric current flowing between the working electrode and the counter electrode. The potentiostat comprises a working electrode terminal for connection to the working electrode, a reference electrode terminal for connection to the reference electrode, and a counter electrode terminal for connection to the counter electrode. The evaluation unit monitors the electric potential of the counter electrode and generates a malfunction signal when said potential is outside a specified reference range. A method of operation is also disclosed.