execute-processor java.lang.NullPointerException

Hello,

I am trying to use execute processor with this example, but I get java.lang.NullPointerException.
Attached full error log.

Thanks in advance

<p:config xmlns:p="http://www.orbeon.com/oxf/pipeline"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xmlns:oxf="http://www.orbeon.com/oxf/processors">

    <!-- Execute command -->
    <p:processor name="oxf:execute-processor">
        <p:input name="config">
            <exec executable="env"/>
        </p:input>
        <p:output name="stdout" id="stdout"/>
        <p:output name="stderr" id="stderr"/>
        <p:output name="result" id="result"/>
    </p:processor>

    <!-- Output stdout -->
    <p:processor name="oxf:http-serializer">
        <p:input name="config">
            <config/>
        </p:input>
        <p:input name="data" href="#stdout"/>
    </p:processor>

</p:config>

Alessandro