Invalid serialnumbers reference key

When I try to create an item fulfillment, I get an Invalid serialnumbers reference key error, even if the serialnumber that I provide is not used up.

I tried setting the "ignoreReadOnlyFields" preference in the header to false, but still getting the same error.

Has anyone faced this issue?

 <soapenv:Envelope xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">      <soapenv:Header>          <ns1:preferences soapenv:mustUnderstand="0" soapenv:actor="http://schemas.xmlsoap.org/soap/actor/next" xmlns:ns1="urn:messages.platform.webservices.netsuite.com">              <ns2:ignoreReadOnlyFields xmlns:ns2="urn:messages_2011_2.platform.webservices.netsuite.com">false</ns2:ignoreReadOnlyFields>          </ns1:preferences>      </soapenv:Header>      <soapenv:Body>          <add xmlns="urn:messages_2011_2.platform.webservices.netsuite.com">              <record xsi:type="ns3:ItemFulfillment" xmlns:ns3="urn:sales_2011_2.transactions.webservices.netsuite.com">                  <ns3:createdFrom xsi:type="ns4:RecordRef" internalId="xxx" xmlns:ns4="urn:core_2011_2.platform.webservices.netsuite.com">                      <ns4:name xsi:type="xsd:string">Sales Order #xxx</ns4:name>                  </ns3:createdFrom>                  <ns3:packageList xsi:type="ns3:ItemFulfillmentPackageList" replaceAll="false">                      <ns3:package xsi:type="ns3:ItemFulfillmentPackage">                          <ns3:packageWeight xsi:type="xsd:double">1.0</ns3:packageWeight>                          <ns3:packageTrackingNumber xsi:type="xsd:string">xxx</ns3:packageTrackingNumber>                      </ns3:package>                  </ns3:packageList>                  <ns3:itemList xsi:type="ns3:ItemFulfillmentItemList" replaceAll="false">                      <ns3:item xsi:type="ns3:ItemFulfillmentItem">                          <ns3:itemReceive xsi:type="xsd:boolean">true</ns3:itemReceive>                          <ns3:description xsi:type="xsd:string">xxx</ns3:description>                          <ns3:location xsi:type="ns5:RecordRef" internalId="4" xmlns:ns5="urn:core_2011_2.platform.webservices.netsuite.com">                              <ns5:name xsi:type="xsd:string">xxx</ns5:name>                          </ns3:location>                          <ns3:quantity xsi:type="xsd:double">1.0</ns3:quantity>                          <ns3:serialNumbers xsi:type="xsd:string">xxx</ns3:serialNumbers>                          <ns3:item xsi:type="ns6:RecordRef" internalId="718" xmlns:ns6="urn:core_2011_2.platform.webservices.netsuite.com">                              <ns6:name xsi:type="xsd:string">xxx</ns6:name>                          </ns3:item>                          <ns3:orderLine xsi:type="xsd:long">1</ns3:orderLine>                      </ns3:item>                  </ns3:itemList>              </record>          </add>      </soapenv:Body>  </soapenv:Envelope>