Newbie: I can't register hosts?

[nic gna]

This is my host.cfg and I can't register the hosts.
  I get this error:
   
  Error: Host name, alias, or address is NULL
Error: Could not register host (config file '/etc/nagios/hosts.cfg', starting on line 1)
  --------
  define host{
        name                    routers
        use                     routers
        check_command           check-host-alive
        max_check_attempts      15
        notification_interval   30
        notification_period     24x7
        contact_groups          admins
        notification_options    d,r
        register                1
        }
define host{
        name                    frontends
        use                     frontends
        check_command           check-host-alive
        max_check_attempts      15
        notification_interval   30
        notification_period     24x7
        contact_groups          admins
        notification_options    d,r
        register                1
        }
define host{
        use                     frontends
        host_name          luke 
        alias                   luke
        address                 129.11.4.169
        }
define host{
        use                     computes
        host_name             leia
          alias                    leia
        address                 129.11.4.170
        }

 
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