diff --git a/zabbix_scripts/check_unifi b/zabbix_scripts/check_unifi
index ed6c4f0..a049b7b 100755
--- a/zabbix_scripts/check_unifi
+++ b/zabbix_scripts/check_unifi
@@ -136,7 +136,7 @@ if ($unifi){
     $json->{$_} = $obj->{$_} if (defined $obj->{$_});
   }
   # Convert last seen into a relative time
-  $json->{last_seen} = time - $obj->{last_seen};
+  $json->{last_seen} = (defined $obj->{last_seen}) ? time - $obj->{last_seen} : time;
   # Add some more info in sys_stats
   $json->{sys_stats}->{$_} = $obj->{'system-stats'}->{$_} foreach (qw/cpu mem uptime/);
 
@@ -199,7 +199,7 @@ if ($unifi){
     $json->{$_} = $obj->{$_} || 0;
   }
   # Convert last_seen to relative
-  $json->{last_seen} = time - $json->{last_seen};
+  $json->{last_seen} = (defined $obj->{last_seen}) ? time - $obj->{last_seen} : time;
 
   # For wireless stations, we gather some more info
   if ($obj->{is_wired} == JSON::false){