ATP700 dhcp issue

Hi all,
i have a problem with an ATP700 i've configured in a school.
Because i've a lot of client i configured the lan interface with ip 192.168.41.254/23 (255.255.254.0) and set dhcp to start from 192.168.40.51 and dhcp pool size 450 client (until 192.168.41.246)
The first ip assigned by the firewall to a client is 192.168.41.0 ! this is an invalid ip and obviously the client don't work, i don't understand why the firewall assign it

I tried also with another atp700 and the result is the same