disabling/fixing "disabling multicast to unicast failed" errors ?

[PGNet Dev]

I've compiled/installed

	hostapd -v
		hostapd v2.7-devel-hostap_2_6-871-g0b287a2+
		User space daemon for IEEE 802.11 AP management,
		IEEE 802.1X/WPA/WPA2/EAP/RADIUS Authenticator
		Copyright (c) 2002-2017, Jouni Malinen <j at w1.fi> and contributors

On exec, it works -- I've got connectivity, etc.

Logs, however, show instances of "disabling multicast to unicast failed" at each start/stop,

	Mar 29 09:00:46 test hostapd[27104]: wlp0s31d1u3: interface state ENABLED->DISABLED
	Mar 29 09:00:46 test hostapd: wlp0s31d1u3: STA 10:31:d4:c3:22:a4 MLME: MLME-DEAUTHENTICATE.indication(10:31:d4:c3:22:a4, 1)
	Mar 29 09:00:46 test hostapd[27104]: wlp0s31d1u3: AP-STA-DISCONNECTED 10:31:d4:c3:22:a4
!!	Mar 29 09:00:46 test hostapd[27104]: nl80211: disabling multicast to unicast failed with -16 (Device or resource busy) on interface wlp0s31d1u3
	Mar 29 09:00:46 test hostapd[27104]: wlp0s31d1u3: AP-DISABLED
	Mar 29 09:00:46 test hostapd[27104]: wlp0s31d1u3: CTRL-EVENT-TERMINATING
	Mar 29 09:00:46 test hostapd[27104]: nl80211: deinit ifname=wlp0s31d1u3 disabled_11b_rates=0
	Mar 29 09:00:47 test hostapd[5894]: Configuration file: /etc/hostapd/hostapd.conf
	Mar 29 09:00:47 test hostapd[5894]: wlp0s31d1u3: interface state UNINITIALIZED->COUNTRY_UPDATE
	Mar 29 09:00:47 test hostapd[5894]: Using interface wlp0s31d1u3 with hwaddr 02:d1:ff:34:15:10 and ssid "testWiFi"
!!	Mar 29 09:00:47 test hostapd[5894]: nl80211: disabling multicast to unicast failed with -16 (Device or resource busy) on interface wlp0s31d1u3
	Mar 29 09:00:47 test hostapd[5894]: wlp0s31d1u3: interface state COUNTRY_UPDATE->ENABLED
	Mar 29 09:00:47 test hostapd[5894]: wlp0s31d1u3: AP-ENABLED

I've tried

	multicast_to_unicast=0
	multicast_to_unicast=1

& 

	 #multicast_to_unicast=0

in the config file -- no change.

What config or build change is required to fix the error -- either in logs, &/or the root cause?