diff --git a/sys/dev/rtwn/if_rtwn_ridx.h b/sys/dev/rtwn/if_rtwn_ridx.h
--- a/sys/dev/rtwn/if_rtwn_ridx.h
+++ b/sys/dev/rtwn/if_rtwn_ridx.h
@@ -95,7 +95,9 @@
 	case 4:		return 1;
 	case 11:	return 2;
 	case 22:	return 3;
-	default:	return RTWN_RIDX_UNKNOWN;
+	default:
+		printf("%s: called; unknown rate (%d)\n", __func__, rate);
+		return RTWN_RIDX_UNKNOWN;
 	}
 }
 
@@ -108,10 +110,23 @@
 	/* Check if we are using MCS rate. */
 	KASSERT(RTWN_RATE_IS_HT(ridx), ("bad mcs rate index %d", ridx));
 
-	rate = (ridx - RTWN_RIDX_HT_MCS(0)) | IEEE80211_RATE_MCS;
+	rate = RTWN_RIDX_TO_MCS(ridx) | IEEE80211_RATE_MCS;
 	cix = rt->info[rt->rateCodeToIndex[rate]].ctlRateIndex;
 	KASSERT(cix != (uint8_t)-1, ("rate %d (%d) has no info", rate, ridx));
 	return rt->info[cix].dot11Rate;
 }
 
+/* VHT version of rtwn_ctl_mcsrate */
+/* XXX TODO: also should move this to net80211 */
+static __inline__ uint8_t
+rtwn_ctl_vhtrate(const struct ieee80211_rate_table *rt, uint8_t ridx)
+{
+
+	/* Check if we are using VHT MCS rate. */
+	KASSERT(RTWN_RATE_IS_VHT(ridx), ("bad mcs rate index %d", ridx));
+
+	/* TODO: there's no VHT tables, so for now just stick to OFDM12 */
+	return 24;
+}
+
 #endif	/* IF_RTWN_RIDX_H */