--- a/ath/if_ath.c
+++ b/ath/if_ath.c
@@ -2723,6 +2723,9 @@ ar_device(int devid)
 static int
 ath_set_ack_bitrate(struct ath_softc *sc, int high)
 {
+	if (!sc->sc_ackrate_override)
+		return 0;
+
 	if (ar_device(sc->devid) == 5212 || ar_device(sc->devid) == 5213) {
 		/* set ack to be sent at low bit-rate */
 		/* registers taken from the OpenBSD 5212 HAL */
@@ -10789,8 +10792,13 @@ ATH_SYSCTL_DECL(ath_sysctl_halparam, ctl
 				break;
 #endif
 			case ATH_ACKRATE:
-				sc->sc_ackrate = val;
-				ath_set_ack_bitrate(sc, sc->sc_ackrate);
+				if (val == -1)
+					sc->sc_ackrate_override = 0;
+				else {
+					sc->sc_ackrate_override = 1;
+					sc->sc_ackrate = val;
+					ath_set_ack_bitrate(sc, sc->sc_ackrate);
+				}
 				break;
 			case ATH_RP:
 				ath_rp_record(sc,
--- a/ath/if_athvar.h
+++ b/ath/if_athvar.h
@@ -681,6 +681,7 @@ struct ath_softc {
 	unsigned int	sc_devstopped:1;	/* stopped due to of no tx bufs */
 	unsigned int	sc_stagbeacons:1;	/* use staggered beacons */
 	unsigned int	sc_dfswait:1;		/* waiting on channel for radar detect */
+	unsigned int	sc_ackrate_override:1;	/* override ack rate */
 	unsigned int	sc_ackrate:1;		/* send acks at high bitrate */
 	unsigned int	sc_dfs_cac:1;		/* waiting on channel for radar detect */
 	unsigned int	sc_hasintmit:1;		/* Interference mitigation */
