The configuration of a midlatitude sporadic-E (Es) layer at a zonal wind-shear node has been assumed to be a stable equilibrium, as long as the wind shear is hydrodynamically stable. In this paper, we show that this equilibrium is in fact unstable, at night, to electrodynamical forces that arise when plane wave perturbations in altitude or density are imposed on the Es layer. We show that the growth rate of the instability depends on the azimuthal alignment of the phase fronts of the plane-wave distortion, a feature that is reminiscient of the Perkins instability <Perkins, 1973>. The dependence on azimuth is consistent with observations of structure in Es layers that show the same preferred alignment.