# File lib/aws/s3/client.rb, line 991
        def require_acl!(acl)
          validate!('acl', acl) do
            case
            when acl.kind_of?(Hash)
              AccessControlList.new(acl).validate!
              nil
            when !acl.respond_to?(:to_str) && !acl.respond_to?(:to_xml)
              "must support to_xml: #{acl.inspect}"
            when acl.nil? || acl == ''
              'may not be blank'
            else
              xml_validation_message(acl)
            end
          end
        end