10.1.3.1 Human Task migration

I'm using single approver with escalation. I want to be able to employ users or groups. Was able to do so without difficulty in 10.1.2..

I have since tried to use an xpath expression to pull the dynamic assignment together even though it can't solve my design goal of optionally choosing user or group. It's frustrating though, the expression builder doesn't work within the task designer and when I cut and paste the appropriate expression I get the error

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

I used an expression builder outside of the task designer to generate the xpath expression. Would assume it was correct. It resolves it outside the task.