I am trying to use the Dynamic User Xpath when assigning a user to a Human Task in my BPEL process. I would like to assign the user based on a field in the input file.  Below is the xpath expression that I use:

bpws:getVariableData('inputVariable','payload','/ns2:Employee/ns2:EmployeeDetail/ns2:UserID')

When I run the BPEL process I get the following error:

Error in evaluating assignees. The error is XPath expression failed to execute.
Error while processing xpath expression, the expression is "bpws:getVariableData('inputVariable','payload','/ns2:Employee/ns2:EmployeeDetail/ns2:UserID')", the reason is message can't be null.
Please verify the xpath query.

Any suggestion as to what the issue might be?
Thank you kindly!