The rate nu at which negative ions nucleate charged vortex rings in isotopically pure superfluid helium-4 has been measured for electric fields of up to 2.5 x 10^6 V/m with pressures and temperatures within 17 < P < 25 bars, 0.3 < T < 0.9 K. It is found that the nucleation process is completely quenched (nu = 0) for E > 2 x 10^6 V/m. The implications of this result for theories of the nucleation mechanism are discussed.