XpathContains comparaison failed ... Expecting [], actual was [null]

Hello,

I use assertions field to field to compare the response of a WebService and equivalent data retrieved by SQL (pro version)
I have a worry in the following cases:

SQL response:
<Results>
     <ResultSet fetchSize = "0">
         <Row rowNumber = "1">
...
             <ZIPA_SETTINGS.ZIPA_SET_VALIDATESECONDPART />
...
        </ Row>
     </ ResultSet>
</ Results>

WebService Response:
<soapenv: Envelope xmlns: soapenv = "http://www.w3.org/2003/05/soap-envelope">
    <soapenv: Body>
...
                <ax2488: validation_secondpart />
....
    </ soapenv: Body>
</ soapenv: Envelope>

The assertion is in error when the XML structures are identical.
XPathContains comparison failed for path [//Results[1]/ResultSet[1]/Row[1]/ZIPA_SETTINGS.ZIPA_SET_VALIDATESECONDPART[1]] expecting [], actual Was [null]

Does anyone have an idea?
Thank you in advance,