[en] The effects of using different proton and neutron wave functions to evaluate matrix elements of a charge independent nucleon-nucleon interaction is examined. It is shown that this is predominantly an isovector effect and in the 1p-shell can give rise to large off diagonal matrix elements (> approximately 100 keV). The magnitude of these matrix elements are extremely sensitive to the detailed structure of the single particle wave functions. Until this effect is satisfactorily taken into account it will be difficult to demonstrate the need for a charge symmetry breaking nucleon-nucleon interaction from a measurement of isospin mixing between T = 0 and 1 states. (Auth.)