Helioseismic observations have demonstrated that the mode frequencies of the oscillations in the active regions are higher than those in the quiet regions. The frequency difference have been used to infer the structural differences between active and quiet regions. The inversions are done under the assumption that the entire frequency difference is due to changes in structure caused by magnetic fields. However, the magnetic fields can affect the oscillations directly through the Lorentz force. Thus the inversions will have systematic errors. In order to get a correct estimate of the structural differences we need to quantify the errors that may be introduced. To assess the magnitude of some of these errors, we perform inversions between models with and without magnetic fields. Our examinations should provide insight in interpreting the inversion results.