format does not allow numbers

app/models/global_setting.rb

@@ -24,7 +24,7 @@ class GlobalSetting
       end
     end
     hostnames = [ hostname ]
-    hostnames << hostname2 if hostname2.present?
+    hostnames << backup_hostname if backup_hostname.present?
 
     hash["host_names"] = hostnames
     hash["database"] = db_name

config/discourse_defaults.conf

@@ -46,8 +46,8 @@ db_prepared_statements = true
 # hostname running the forum
 hostname = "www.example.com"
 
-# backup hostname
-hostname2 =
+# backup hostname mainly for cdn use
+backup_hostname =
 
 # address of smtp server used to send emails
 smtp_address =