Invalid sequence type

The pipeline

<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error"
                xmlns:p="http://www.w3.org/ns/xproc"
                xmlns:t="http://xproc.org/ns/testsuite/3.0" name="main" version="3.0">
   <p:output port="result" sequence="true"/>
   <p:variable name="error" as="foo bar baz"
               select="'hello world'"/>
   <p:identity name="inline">
      <p:with-input port="source">
         <p:inline>
            <t:doc>{$error}</t:doc>
         </p:inline>
      </p:with-input>
   </p:identity>
</p:declare-step>