I am reading a proof that the $\vec{E}$-field is constant inside a cylindrical resistor, and I don't understand one of the steps. It is stated that since the surrounding medium is non-conductive the flow of charge at the surface has no component along the normal of the surface. From this the conclusion is drawn that the $\vec{E}$-field along the normal must be zero too.
This I don't understand. Since the conductivity of the surrounding medium is assumed to approach zero couldn't the $\vec{E}$-field be nonzero without causing charge to flow?