BPEL - optional element is required by the assign action

Question

Having an assign step in the BPEL process we map some elements from the variable to a service request (another message type variable)

The element (xml element) according to the schema is optional (may be missing). So when mapping, I assume the attribute ignoreMissingFromData="yes" should ignore the missing source element. But apparently when the element is missing, the engine throws a fault.

<bpel:copy ignoreMissingFromData="yes">
  <bpel:from variable="item">
  <bpel:query queryLanguage="urn:oasis:names:tc:wsbpel:2.0:sublang:xpath1.0">
    <![CDATA[wi:clientRequestData/wi:equipmentID]]>
  </bpel:query>
  </bpel:from>
     <bpel:to part="parameters" variable="esbReq">
     <bpel:query queryLanguage="urn:oasis:names:tc:wsbpel:2.0:sublang:xpath1.0"><![CDATA[ns1:equipmentId]]></bpel:query>
  </bpel:to>
 </bpel:copy>

Is really the "ignoreMissingFromData" attribute ignored or shall I look for the problem somewhere else?

Thanks for any hints