SOAP API: why getMultipleLeads request doesnt return anything?

Hello all.

I want to retrieve the leads from a specific list. I'm sending the following query to the API:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:mkt="http://www.marketo.com/mktows/">
   <soapenv:Header>
      <mkt:AuthenticationHeader>
         <mktowsUserId>*****</mktowsUserId>
         <requestSignature>*****</requestSignature>
         <requestTimestamp>2014-12-02T14:37:24+01:00</requestTimestamp>
      </mkt:AuthenticationHeader>
   </soapenv:Header>
   <soapenv:Body>
   
      <mkt:paramsGetMultipleLeads>
         <!--Optional:-->
        <StaticListSelector>
            <leadSelector xsi:type="ns1:StaticListSelector">
                <staticListName>MyProgram.MyList</staticListName>
          </leadSelector>
      </StaticListSelector>
      <batchSize>20</batchSize>
      </mkt:paramsGetMultipleLeads>
   </soapenv:Body>
</soapenv:Envelope>

But for some reasons it doesnt work. I get this as a result:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns1="http://www.marketo.com/mktows/">
   <SOAP-ENV:Body>
      <ns1:successGetMultipleLeads>
         <result>
            <returnCount>0</returnCount>
            <remainingCount>0</remainingCount>
            <newStreamPosition>id:0:ts:1417527490:os:0:rc:0</newStreamPosition>
            <leadRecordList/>
         </result>
      </ns1:successGetMultipleLeads>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

For information I'm sure my list contains some leads (more than 5000)

Thank you very much for your help