JSON syntax - map_type

I've been playing around with setting up a page using your sample code under the Advanced tag.  Using your demo.json example, I can create a json object on my page, and everything works fine.  I've tried to change the base map type as shown in the following JSON fragment:

       var storymap_data = {"storymap": { "map_type": "stamen:terrain", "slides": [ .....

But the map type remains Stamen toner.  Am I missing something here?  Has the JSON syntax changed? Thanks for your response!