diff --git a/sys/dev/ath/ath_hal/ah.c b/sys/dev/ath/ath_hal/ah.c
index 9a30dba7ef72..574e46cd71b2 100644
--- a/sys/dev/ath/ath_hal/ah.c
+++ b/sys/dev/ath/ath_hal/ah.c
@@ -327,9 +327,9 @@ ath_computedur_ht(uint32_t frameLen, uint16_t rate, int streams,
 	KASSERT((rate &~ IEEE80211_RATE_MCS) < 31, ("bad mcs 0x%x", rate));
 
 	if (isht40)
-		bitsPerSymbol = ht40_bps[rate & 0xf];
+		bitsPerSymbol = ht40_bps[rate & 0x1f];
 	else
-		bitsPerSymbol = ht20_bps[rate & 0xf];
+		bitsPerSymbol = ht20_bps[rate & 0x1f];
 	numBits = OFDM_PLCP_BITS + (frameLen << 3);
 	numSymbols = howmany(numBits, bitsPerSymbol);
 	if (isShortGI)