Error test XS0096 006

The pipeline

<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:p="http://www.w3.org/ns/xproc" xmlns:xs="http://www.w3.org/2001/XMLSchema" version="3.0">
   <p:output port="result">
   <p:add-attribute match="/" attribute-name="a">
      <p:with-input port="source">
         <doc/>
      </p:with-input>
      <p:with-option name="attribute-value" as="foo" select="5">
   </p:add-attribute>
</p:declare-step>