ocserv trying to assign IP address 255.255.255.254 to tun device causes authentication failed

[syouwa]

Freeradius is my authentication method, I found that ocserv trying to 
assign IP address 255.255.255.254 to tun device and seems that caused 
authentication fail, 255.255.255.254 is the value of Framed-IP-Address 
attribute defined in radgoupreply table, is this a bug?

...
ocserv[6517]: radius-auth: opening session 
QEZrDavGuU+alu9EEOX7WGVCXl/kRtD0iD9rZAPEGY8=
ocserv[6517]: sec-mod: initiating session for user 'syouwa at gmail.com' 
(session: QEZrDa)
ocserv[6516]: main[syouwa at gmail.com]: 111.202.52.130:50127 new user session
ocserv[6516]: main[syouwa at gmail.com]: 111.202.52.130:50127 assigned 
IPv4: 255.255.255.254
ocserv[6516]: main[syouwa at gmail.com]: 111.202.52.130:50127 assigning tun 
device vpns0
ocserv[6516]: main: tun.c:386: vpns0: Error setting DST IPv4: Invalid 
argument
ocserv[6516]: main[syouwa at gmail.com]: 111.202.52.130:50127 failed 
authentication attempt for user 'syouwa at gmail.com'
...


 From rfc2865 Framed-IP-Address definition

...
    Address

       The Address field is four octets.  The value 0xFFFFFFFF indicates
       that the NAS Should allow the user to select an address (e.g.
       Negotiated).  The value 0xFFFFFFFE indicates that the NAS should
       select an address for the user (e.g. Assigned from a pool of
       addresses kept by the NAS).  Other valid values indicate that the
       NAS should use that value as the user's IP address.
...


Thanks,
David