A method of determining a zero point of a current sensor in a circuit arrangement for operating a gas discharge lamp includes switching off the current through the sensor for a short period during a first half wave and determining a first test value. Then the current through the sensor is switched off for a short period during a second half wave having a different polarity and a second test value is determined. The average value of the two test values is formed and the zero point is determined using the average value. It is prevented thereby that the zero point drifts during lamp operation, and that amplitudes of the positive and negative half waves of the lamp current are formed differently.