BPEL eclipse "assign" initialize

Question

I have been working on eclipse for WS-BPEL processes. In a basic helloworld.bpel process, an assign block (which is between receiveInput and replyOutput blocks)requires input and output validation. But once I set input variable to input->payload->input and output to output->payload->result, I get this error

eObject cannot be null in getNamespaceMap()

Can anyone tell what this means and how I can deal with it?

Thanks

<!-- helloworld BPEL Process [Generated by the Eclipse BPEL Designer] -->
<bpel:process name="helloworld"
     targetNamespace="http://helloworld"
     suppressJoinFailure="yes"
     xmlns:tns="http://helloworld"
     xmlns:bpel="http://docs.oasis-open.org/wsbpel/2.0/process/executable"
     expressionLanguage="http://www.w3.org/TR/1999/REC-xpath-19991116">

<!-- Import the client WSDL -->
<bpel:import location="helloworldArtifacts.wsdl" namespace="http://helloworld" 
        importType="http://schemas.xmlsoap.org/wsdl/" />

<!-- ================================================================= -->         
<!-- PARTNERLINKS                                                      -->
<!-- List of services participating in this BPEL process               -->
<!-- ================================================================= -->         
<bpel:partnerLinks>
    <!-- The 'client' role represents the requester of this service. -->
    <bpel:partnerLink name="client"
                 partnerLinkType="tns:helloworld"
                 myRole="helloworldProvider"
                 />
</bpel:partnerLinks>

<!-- ================================================================= -->         
<!-- VARIABLES                                                         -->
<!-- List of messages and XML documents used within this BPEL process  -->
<!-- ================================================================= -->         
<bpel:variables>
    <!-- Reference to the message passed as input during initiation -->
    <bpel:variable name="input"
              messageType="tns:helloworldRequestMessage"/>

    <!-- 
      Reference to the message that will be returned to the requester
      -->
    <bpel:variable name="output"
              messageType="tns:helloworldResponseMessage"/>
</bpel:variables>

<!-- ================================================================= -->         
<!-- ORCHESTRATION LOGIC                                               -->
<!-- Set of activities coordinating the flow of messages across the    -->
<!-- services integrated within this business process                  -->
<!-- ================================================================= -->         
<bpel:sequence name="main">

    <!-- Receive input from requester. 
         Note: This maps to operation defined in helloworld.wsdl 
         -->
    <bpel:receive name="receiveInput" partnerLink="client"
             portType="tns:helloworld"
             operation="process" variable="input"
             createInstance="yes"/>

    <!-- Generate reply to synchronous request -->



    <bpel:assign validate="no" name="Assign">


        <bpel:copy>
            <bpel:from part="payload" variable="input">
                <bpel:query queryLanguage="urn:oasis:names:tc:wsbpel:2.0:sublang:xpath1.0">
                    <![CDATA[tns:input]]>
                </bpel:query>
            </bpel:from>
            <bpel:to part="payload" variable="output">
                <bpel:query queryLanguage="urn:oasis:names:tc:wsbpel:2.0:sublang:xpath1.0">
                    <![CDATA[tns:result]]>
                </bpel:query>
            </bpel:to>
        </bpel:copy>
    </bpel:assign>
    <bpel:reply name="replyOutput" 
           partnerLink="client"
           portType="tns:helloworld"
           operation="process" 
           variable="output"
           />
</bpel:sequence>