Index: head/sys/net80211/ieee80211_output.c
===================================================================
--- head/sys/net80211/ieee80211_output.c
+++ head/sys/net80211/ieee80211_output.c
@@ -849,6 +849,15 @@
 	return (ret);
 }
 
+static void
+ieee80211_nulldata_transmitted(struct ieee80211_node *ni, void *arg,
+    int status)
+{
+	struct ieee80211vap *vap = ni->ni_vap;
+
+	wakeup(vap);
+}
+
 /*
  * Send a null data frame to the specified node.  If the station
  * is setup for QoS then a QoS Null Data frame is constructed.
@@ -937,6 +946,11 @@
 		    vap->iv_opmode != IEEE80211_M_HOSTAP)
 			wh->i_fc[1] |= IEEE80211_FC1_PWR_MGT;
 	}
+	if ((ic->ic_flags & IEEE80211_F_SCAN) &&
+	    (ni->ni_flags & IEEE80211_NODE_PWR_MGT)) {
+		ieee80211_add_callback(m, ieee80211_nulldata_transmitted,
+		    NULL);
+	}
 	m->m_len = m->m_pkthdr.len = hdrlen;
 	m->m_flags |= M_ENCAP;		/* mark encapsulated */
 
Index: head/sys/net80211/ieee80211_scan_sw.c
===================================================================
--- head/sys/net80211/ieee80211_scan_sw.c
+++ head/sys/net80211/ieee80211_scan_sw.c
@@ -629,13 +629,9 @@
 		if ((vap->iv_bss->ni_flags & IEEE80211_NODE_PWR_MGT) == 0) {
 			/* Enable station power save mode */
 			vap->iv_sta_ps(vap, 1);
-			/*
-			 * Use an 1ms delay so the null data frame has a chance
-			 * to go out.
-			 * XXX Should use M_TXCB mechanism to eliminate this.
-			 */
+			/* Wait until null data frame will be ACK'ed */
 			mtx_sleep(vap, IEEE80211_LOCK_OBJ(ic), PCATCH,
-			    "sta_ps", msecs_to_ticks(1));
+			    "sta_ps", msecs_to_ticks(10));
 			if (ss_priv->ss_iflags & ISCAN_ABORT) {
 				scan_done(ss, 0);
 				return;