Invoking a process to return a SQL query as XML results in empty XML tag (but testing the SQL works)

I have a process that runs a SQL query and returns the results as XML.  When I test the query in the Process Properties tab in Workbench it appears to execute just fine.  I can also test the XML information and see that the results are coming back correctly.  But when I invoke the process I get an emtpy XML tag with no results.  Recording the invocation and playing back the recording doesn't tell me anything useful.  Has anyone ever seen this issue before?  I don't understand why everything within the process seems to bring back results just fine but invoking it returns nothing.