diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index d6669fd..003e4a0 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -330,6 +330,7 @@
 
 	union {
 		struct {
+			/* NB: vif can be NULL for injected frames */
 			struct ieee80211_vif *vif;
 			struct ieee80211_key_conf *hw_key;
 			struct ieee80211_sta *sta;