No WEP wireless connection

Xubuntu Gutsy
Linux xubuntu 2.6.22-14-generic #1 SMP Tue Dec 18 08:02:57 UTC 2007 i586 GNU/Linux

My Wireless card is a RaLink RT61

With no WEP, the conection works fine:
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sudo iwconfig wlan0 mode managed ;
sudo ifconfig wlan0 up ;
sudo iwconfig wlan0 essid Chronos ;

sudo iwconfig wlan0 key off ;

sudo dhclient wlan0
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When trying to work under WEP I get:

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acardh@xubuntu:~$ sudo iwconfig wlan0 key restricted
Error for wireless request "Set Encode" (8B2A) :
    SET failed on device wlan0 ; Invalid argument.
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