SQL in create_admin_account

First time creating admin account I got an error. Id is not INT.
$sql="insert into ".$this->prefix."users values ('','".mysql_real_escape_string(htmlentities($firstname,ENT_QUOTES))."','".mysql_real_escape_string(htmlentities($lastname,ENT_QUOTES))."','".mysql_real_escape_string($email)."','1',".mktime().")";

Place of Id here is '' string, so I change it to null.
After this it worked.

I dont know its my system, or its something else.

I run it on localhost with WAMP.

Thanks for letting me know! I haven't seen that, but it certainly would be good practice to use null in an int field rather than an empty string! Good call, thanks. :)
I'll make the change eventually,
Bob