The weird thing is that you're stubbing out a MustacheNode with a POJO. If you actually were to create a MustacheNode, the MustacheNode constructor handles the backwards compat stuff. So maybe we shouldn't stub MustacheNode but actually use the constructor? (and then create two tests, one with the old form of passing in the params/hash to the mustacheNode and the new one which passes in a SexprNode)