IAPP weirdness

[IO te]

Hi all,
I'm trying to use IAPP with hostapd 0.5.7 and MadWiFi on IXP425
boards. My hostapd configuration is written below:
interface=ath1
driver=madwifi
logger_syslog=-1
logger_syslog_level=2
logger_stdout=-1
logger_stdout_level=1
ctrl_interface=/var/run/hostapd
ctrl_interface_group=0
ssid=provaiapp
auth_algs=3
wpa=1
wpa_key_mgmt=WPA-PSK
wpa_pairwise=CCMP TKIP
eap_server=0
wpa_passphrase=iappiapp
iapp_interface=eth0

When I execute hostapd I get the following error:
ath1: IEEE 802.11 Fetching hardware channel/rate support not supported.
Using interface ath1 with hwaddr <ap_mac> and ssid 'provaiapp'
setsockopt[UDP,IP_ADD_MEMBERSHIP]: No such device
setsockopt[UDP,IP_DEL_MEMBERSHIP]: No such device
IEEE 802.11F (IAPP) initialization failed.

Note that the same configuration works without the statement 'iapp_interface'.

What's the problem?

Thank you for your help and sorry for my english...

Massimiliano