Q: 802.11 vap monitor mode

During testing a new driver, I got a problem to test a monitor mode at
802.11 vap I'm using.

# uname -a
FreeBSD kkk 8.0-CURRENT FreeBSD 8.0-CURRENT #6: Wed May 14 11:27:37 KST
2008     root_at_kkk:/usr/obj/usr/src/sys/GENERIC  i386
# ifconfig wlan0 create wlandev upgt0 wlanmode monitor
# ifconfig wlan0 up
# ifconfig
[...]
upgt0:
flags=108943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST,NEEDSGIANT>
metric 0 mtu 2290
        ether 00:0f:b5:dd:6f:36
        media: IEEE 802.11 Wireless Ethernet autoselect mode 11g
<monitor>
        status: running
wlan0:
flags=28943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST,PPROMISC>
metric 0 mtu 1500
        ether 00:0f:b5:dd:6f:36
        inet6 fe80::20f:b5ff:fedd:6f36%wlan0 prefixlen 64 scopeid 0xd 
        media: IEEE 802.11 Wireless Ethernet autoselect mode 11g
<monitor>
        status: running
        ssid "" channel 1 (2412 Mhz 11g) bssid 00:0f:b5:dd:6f:36
        country US authmode OPEN privacy OFF txpower 0 scanvalid 60
        protmode CTS

As seeing, it looks work but the problem is happend when I ran stumbler
(src/tools/tools/net80211/stumbler) program.

The error message is "ioctl(BIOCSDLT): Invalid argument" that it's
emitted to screen when I execute the following command:
# ./stumbler wlan0

Are there something I missed?

regards,
Weongyo Jeong