diff --git a/sys/net80211/ieee80211.c b/sys/net80211/ieee80211.c
index 699728ee6909..a36abf1aa0d9 100644
--- a/sys/net80211/ieee80211.c
+++ b/sys/net80211/ieee80211.c
@@ -1386,6 +1386,8 @@ getflags(const uint8_t bands[], uint32_t flags[], int ht40, int vht80)
 
 /*
  * Add one 20 MHz channel into specified channel list.
+ * You MUST NOT mix bands when calling this.  It will not add 5ghz
+ * channels if you have any B/G/N band bit set.
  */
 /* XXX VHT */
 int