Co-continuous morphologies in a blend of poly(ether-ester) and polystyrene break up into a droplet/matrix morphology when subjected to a shear flow in a rheometer with a cone-plate configuration and are therefore not stable under shear. It is shown that the capillary number at these flow conditions is too small to maintain stable extended structures that are necessary for a co-continuous morphology. Go-continuity at such flow conditions is limited to the point of phase inversion, contrary to the range of co-continuity usually found after extrusion.