diff --git a/sys/dev/ath/if_ath.c b/sys/dev/ath/if_ath.c
index 10a1b6d9c82..1369f933de5 100644
--- a/sys/dev/ath/if_ath.c
+++ b/sys/dev/ath/if_ath.c
@@ -1725,6 +1725,7 @@ ath_tx_start(struct ath_softc *sc, struct ieee80211_node *ni, struct ath_buf *bf
 			return ENOMEM;
 		}
 		ivp = hdrbuf + hdrlen;
+		wh = mtod(m0, struct ieee80211_frame *);
 		/*
 		 * XXX
 		 * IV must not duplicate during the lifetime of the key.