trouble with port in iteration

Using Calabash.

I have following try/catch which works both for valid and non-valid  
docs.

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" xmlns:c="http://www.w3.org/ns/xproc-step 
">
     <p:try>
         <p:group>
             <p:validate-with-xml-schema assert-valid="true"  
mode="strict">
                 <p:input port="source">
                     <p:document
                         href="file:///Users/paul/Desktop/R&amp;D/ 
xmlschema/msData/simpleType/stE056.xml"
                     />
                 </p:input>
                 <p:input port="schema">
                     <p:document
                         href="file:///Users/paul/Desktop/R&amp;D/ 
xmlschema/msData/simpleType/stE055.xsd"
                     />
                 </p:input>
             </p:validate-with-xml-schema>
             <p:identity>
                 <p:input port="source">
                     <p:inline><c:result>valid</c:result></p:inline>
                 </p:input>
             </p:identity>
         </p:group>
         <p:catch>
             <p:identity>
                 <p:input port="source">
                     <p:inline><c:result>invalid</c:result></p:inline>
                 </p:input>
             </p:identity>
         </p:catch>
     </p:try>
     <p:store
         href="/Users/paul/Desktop/R&amp;D/xmlschema/msData/simpleType/ 
stE055.res"
         name="store"/>
</p:declare-step>

Now I'm doing the same within a loop

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc"
     xmlns:wts="http://www.w3.org/XML/2004/xml-schema-test-suite/"
     xmlns:c="http://www.w3.org/ns/xproc-step" xmlns:xlink="http://www.w3.org/1999/xlink 
"
     xpath-version="2.0">
     <p:output port="result" sequence="true">
         <p:pipe port="result" step="loop"/>
     </p:output>
     <p:load href="/Users/paul/Desktop/R&amp;D/xmlschema/msMeta/ 
SimpleType_w3c.xml"/>
     <p:for-each name="loop">
         <p:iteration-source select="/wts:testSet/ 
wts:testGroup[wts:instanceTest/wts:instanceDocument]"/>
         <p:output port="result" sequence="true">
             <p:pipe port="result" step="write2file"/>
         </p:output>
         <p:variable name="xsd"
             select="substring-after(/wts:testGroup/wts:schemaTest/ 
wts:schemaDocument/@xlink:href,'/')"/>
         <p:variable name="xml"
             select="substring-after(/wts:testGroup/wts:instanceTest/ 
wts:instanceDocument/@xlink:href,'/')"/>
         <p:load name="schema">
             <p:with-option select="$xsd" name="href"
                 xml:base="/Users/paul/Desktop/R&amp;D/xmlschema/"/>
         </p:load>
         <p:load name="instance">
             <p:with-option select="$xml" name="href"
                 xml:base="/Users/paul/Desktop/R&amp;D/xmlschema/"/>
         </p:load>
         <p:try name="try">
             <p:group>
                 <p:validate-with-xml-schema assert-valid="true"  
mode="strict">
                     <p:input port="source">
                         <p:pipe port="result" step="instance"/>
                     </p:input>
                     <p:input port="schema">
                         <p:pipe port="result" step="schema"/>
                     </p:input>
                 </p:validate-with-xml-schema>
                 <p:identity>
                     <p:input port="source">
                         <p:inline>
                             <c:result>valid</c:result>
                         </p:inline>
                     </p:input>
                 </p:identity>
             </p:group>
             <p:catch name="catch">
                 <p:identity>
                     <p:input port="source">
                         <p:inline>
                             <c:result>invalid</c:result>
                         </p:inline>
                     </p:input>
                 </p:identity>
             </p:catch>
         </p:try>
          <p:store name="write2file">
              <p:with-option name="href" select="concat('/Users/paul/ 
Desktop/R&amp;D/xmlschema/',$xml,'.err')"/>
         </p:store>
     </p:for-each>
</p:declare-step>

As long as all files are valid, I do get the required output.
Once I encounter a non-valid file, I get following error:

Pipeline failed: err:XD0007: null  If sequence is not specified on  
p:output, or has the value false, then it is a dynamic error if the  
step does not produce exactly one document on the declared port.
Something related to the error output on the catch?


Kind Regards,
Paul Hermans

-------------------------