wpa_supplicant receive "Wireless event: new AP: 00:00:00:00:00:00" event?

[Wesley]

I set up my enviroment with hostap devel branch (snapshot-20040405).
After wpa_supplicant associate with hostapd and finish
four-way-handshake, all the packet can be sent normally. then I run
'iwlist wlan0 scan' peridocally. Usually (after three or more scans)
the wpa_supplicant disiplay "Wireless event: new AP:
00:00:00:00:00:00" message and do Disassociation and network blocked.
Does somebody know what cause this "new AP event" occur?

thanks a lot

wesley