hostpad failed when station active

[Bob]

Hello list,

I am trying to enable software AP in linux along with wifi station. The 
compiled 4.4.13 kernel already provides the required drivers

~~~
#lsmod | grep 80211

mac80211    630784  1 ath9k
cfg80211      548864  4 ath,ath9k_common,ath9k,mac80211
~~~


No issue to start the AP alone with the minimal hostapd configuration

~~~
interface=sw_ap
driver=nl80211
ssid=test
channel=6
wmm_enabled=1
hw_mode=g
~~~

but failed when virtual station is already up. Then hostapd gives 
following errors

~~~
nl80211: send_mlme - da= ff:ff:ff:ff:ff:ff noack=0 freq=0 no_cck=0 
offchanok=0 wait_time=0 fc=0xc0 (WLAN_FC_STYPE_DEAUTH) nlmode=3
nl80211: send_mlme -> send_frame
nl80211: send_frame - Use bss->freq=2412
nl80211: send_frame -> send_frame_cmd
nl80211: Frame command failed: ret=-16 (Device or resource busy) 
(freq=2412 wait=0)
~~~~

"iw list" suggests

```
     Supported interface modes:
          * IBSS
          * managed
          * AP
          * AP/VLAN
          * WDS
          * monitor
          * mesh point
          * P2P-client
          * P2P-GO
          * Unknown mode (11)
````

Have I missed anything ?
hostapd   version 2.3

regards,
Bob