<< instead of +=

app/helpers/application_settings_helper.rb

@@ -65,7 +65,7 @@ module ApplicationSettingsHelper
     button_based_providers.map do |source|
       disabled = current_application_settings.disabled_oauth_sign_in_sources.include?(source.to_s)
       css_class = 'btn'
-      css_class += ' active' unless disabled
+      css_class << ' active' unless disabled
       checkbox_name = 'application_setting[enabled_oauth_sign_in_sources][]'
 
       label_tag(checkbox_name, class: css_class) do