Admittance calculation of a circuit

I am measuring the admittance of a circuit, by switching a load off and on at different loads. The admittance value I got from the measured voltage drop and current flowing through my circuit. The problem is calculation is only good when the circuit is only resistive or having high power factor (0.9 to 1.0). As soon as the inductive impedance value goes up, the admittance measurement shows discrepancies. Can somebody please throw some light on it?