simple transformation: xmlns="" in output

Hey,

I created this simple transormation Z1

<?sap.transform simple?>
<tt:transform xmlns:tt="http://www.sap.com/transformation-templates">

  <tt:root name="REPORT"/>
  <tt:root name="R0390"/>

  <tt:template>
    <cssr_document xmlns="http://www.nbb.be/cssr">

      <content xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CPAAcquisitionDataset">
        <survey> <tt:attribute name="code" value-ref="report"/> 
          <form code="0390">
            <tt:call transformation="Z2">
              <tt:with-root name="ROOT" ref="R0390"/>
            </tt:call>
          </form>
        </survey>
      </content>
    </cssr_document>
  </tt:template>
</tt:transform>

that as you see calls another transformation Z2:

<?sap.transform simple?>
<tt:transform xmlns:tt="http://www.sap.com/transformation-templates" xmlns:ss="cc">


  <tt:root name="ROOT"/>

  <tt:template>

    <tt:loop ref=".ROOT">
      <item>
        <tt:s-cond check="not-initial($REF.Rubriek)" using="exist($REF.Rubriek)">
          <tt:attribute name="C013" value-ref="$REF.Rubriek"/>
        </tt:s-cond>
      </item>
    </tt:loop>

  </tt:template>

</tt:transform>

In the output I the element <item> gets the attribute xmlns="" and I have no idea where it comes from, and worse: I have no idea to remove it...

<?xml version="1.0" encoding="iso-8859-1"?>
<cssr_document xmlns="http://www.nbb.be/cssr">
<survey code="S10INS">
<form code="0390"><item xmlns="" C013="1234" />  </form>
  </survey>
  </content>
  </cssr_document>

Someone an idea how I can avoid this attribute?