Error XD0051 009 (AB)

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" version="3.0">
   <p:output port="result"/>
   <p:variable name="func" select="function($a){$a}"/>
   <p:add-attribute attribute-name="att"
                    attribute-value="{ $func }">
      <p:with-input>
         <doc/>
      </p:with-input>
   </p:add-attribute>
</p:declare-step>