The dielectric anomaly observed at the ferroelastic transition (146 K) in ammonium hydrogen oxalate hemihydrate is shown to be due to biquadratic coupling with an incipient ferroelectric mode of ordering via a common strain. The sign of the coupling constant obtained by fitting to measured data provides evidence in support of the structural resonance theory of the pressure-induced incommensurate phase in this material recently proposed by the same author.