Call as ERB::Util.html_escape since is not the module is not included here

actionpack/lib/action_view/helpers/capture_helper.rb

@@ -39,7 +39,7 @@ module ActionView
         value = nil
         buffer = with_output_buffer { value = yield(*args) }
         if string = buffer.presence || value and string.is_a?(String)
-          html_escape string
+          ERB::Util.html_escape string
         end
       end