Handling JSON in Presto

I am creating a mashup using the REST services published in presto. If I keep the response type as XML for these services, I am able to create a mashup with multiple joins and filter conditions and run it fine.

However, my project requirement is to return the json string as REST service response and not the XML.
With json response type of these REST services, I don't get any output in presto wires preview of these 
services. 

I also tried mashup studio for creating EMML files. I am able to create the mashup and run it fine as long as 
rest services response is in xml format but not with JSON (with JSON no output again). 

I have checked my services and the JSON response is fine and is parsable with other tools. Am I missing anything with handling JSON in Presto? Strangely, while publishing the rest service when I try to copy
sample JSON response in "Sample XML/JSON or Response Schema URL:" field, it returns an error saying only XML or XML schema is allowed.