Can't pass variable text into a template for use in a Planning expression

Summary:

I have a groovy rule that executes a calc that references a Template. Here is a snippet:

String ApproverMsg = '"' + "This is a Test" + '"'

Calc ="""
  %Template(name:="promotion",application:="testapp",plantype:="testdb",
     dtps:=(  
         "ApproveMsg":=[[$ApproverMsg]]
         ))
 """;

In the Template referenced, I use a design time prompt (of type string passed in from the rule above) in a Planning expression like so:

"Approver" = [[PlanningFunctions.getIdForString([ApproveMsg])]];

Unfortunately it throws an error on execution. If however I hardcode the DTP in the Template call instead of using a variable it works fine:

 Calc ="""
 %Template(name:="prmotion",application:="testapp",plantype:="testdb",
   dtps:=(  
     "ApproveMsg":=[["This is a Test"]]
     ))
 """;