I have the (rather dreaded, it seems) Broadcom 1390 wireless card in my laptop, but unlike many other users, I have had no problems with it. Ubuntu correctly configured it with b43-fwcutter during install, and I have since reinstalled it to be sure. However, while everything I have read tells me that monitor mode should work normally on my card at this point, whenever I try to use airmon-ng to enable monitor mode, I receive:
wlan0                   iwl3945 - [phy0]/usr/sbin/airmon-ng: 833: cannot create /sys/class/ieee80211/phy0/add_iface: Directory nonexistent
Error for wireless request "Set Mode" (8B06) :
    SET failed on device mon0 ; No such device.
mon0: ERROR while getting interface flags: No such device

                                (monitor mode enabled on mon0)
Why is this occurring, and how can I fix it?