The ill-conditioned stable inversion is studied for slightly nonminimum phase systems whose zero dynamics is singularly perturbed,that is,the relative degree is ill-defined.For these systems,we show that there exists an inherent limitation in the bandwidth of a reference trajectory to be tracked when a well-conditioned feedforward input via stable inversion is sought.We assert that,when the violation of this limimtion occurs,the so-called reference trajectory redesign ia called for.Our analysis results can provide an explicit assessment as well as useful guidance for the reference trajectory redesign if needed.