JSON Response

I am trying to get the ASM to send back a JSON response when someone hits a particular uri. I currently have an HTML and SOAP response working with no issues on this ASM. I went in and created the "content profile", and built the parameter, based off the uri. And added it to the "Allowed URL" list. But when someone sends in data that is incorrect, they continue to receive the HTML response page back. Is there something that I am missing here? Any help would be greatly appreciated!