diff --git a/app/models/site_server.rb b/app/models/site_server.rb
index 16a43c9..a1b3e98 100644
--- a/app/models/site_server.rb
+++ b/app/models/site_server.rb
@@ -38,8 +38,8 @@ class SiteServer
     if (need_change || (!need_change && self.enable_redirect_default_domain))
       if self.default_domain_names_changed?
         check_domains = []
-        removed_default_domains = self.default_domain_names_was - self.default_domain_names
-        add_default_domains = self.default_domain_names - self.default_domain_names_was
+        removed_default_domains = self.default_domain_names_was.to_a - self.default_domain_names
+        add_default_domains = self.default_domain_names - self.default_domain_names_was.to_a
         check_domains += removed_default_domains
         check_domains += add_default_domains
         check_domains = check_domains.map{|domain| Regexp.new("(\\A|\\s)"+domain.gsub(".","\\.").gsub("*","[^.]*") + "($|\\s)")}