Error test XS0096 006

The pipeline

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