Error while assigning the value to the repeating element in BPEL 11g

Question

I want to generate the below request for my webservice in BPEL 11g

<!--1 or more repetitions:-->

<dep:KeyValuePair>

    <dep:KEY>AuthenticationString</dep:KEY>

    <dep:VALUE>Basic c2ZhcGlAQWhvbGRUU1Q6UjEyMDBncw==</dep:VALUE>

</dep:KeyValuePair>

            <dep:KeyValuePair>

    <dep:KEY>$filter</dep:KEY>

    <dep:VALUE>lastModifiedOn ge datetime'2014-01-13T18:18:14Z'</dep:VALUE>

</dep:KeyValuePair>

where KeyValuePair is a unbounded element. For this i have taken a assign activity in BPEL 11g and did the following

        <copy>

            <from expression='concat("Basic ",$Base64AuthenticationString)'/>

            <to variable="EncodedString"/>

        </copy>

        <copy>

            <from expression="'AuthenticationString'"/>

            <to variable="InvokeDepartmentDetailsPSReqMsg"

                part="DepartmentProxyRequest"

                query="/sfabmabo:DepartmentProxyRequest/sfabmabo:KeyValuePair[1]/sfabmabo:KEY"/>

        </copy>

        <copy>

            <from expression="bpws:getVariableData('EncodedString')"/>

            <to variable="InvokeDepartmentDetailsPSReqMsg"

                part="DepartmentProxyRequest"

                query="/sfabmabo:DepartmentProxyRequest/sfabmabo:KeyValuePair[1]/sfabmabo:VALUE"/>

        </copy>

        <copy>

            <from expression="'$filter'"/>

            <to variable="InvokeDepartmentDetailsPSReqMsg"

                      part="DepartmentProxyRequest"

                      query="/sfabmabo:DepartmentProxyRequest/sfabmabo:KeyValuePair[2]/sfabmabo:KEY"/>

        </copy>

        <copy>

            <from expression="concat(&quot;lastModifiedOn ge datetime'&quot;,$SuccessFactorTimeStamp,&quot;'&quot;)"/>

            <to variable="InvokeDepartmentDetailsPSReqMsg"

                      part="DepartmentProxyRequest"

                      query="/sfabmabo:DepartmentProxyRequest/sfabmabo:KeyValuePair[2]/sfabmabo:VALUE"/>

        </copy>

    </assign>

but i am getting the below error while doing the same

XPath query string returns zero node. The assign activity of the to node query is returning zero node. Either the to node data or the xpath query in the to node was invalid. According to BPEL4WS spec 1.1 section 14.3, verify the to node value at line number 327 in the BPEL source.

Can anyone please help me to resolve the same.

Thanks