CookieStorage plugin and JSON decode

My current application is using M4 and is working fine.

I just ran a test on the current build from GitHub (RC2) and
encountered an error from the JSON.cfc conversion component.

Error Type: JSON.InvalidJSON : [N/A]
Error Messages: Invalid JSON
The document you are trying to decode is not in valid JSON format

The call was coming from the CookieStorage plugin at the following

143: <cfset rtnVar = instance.json.decode(rtnVar)>

Changing this to:

<cfif isJSON( rtnVar)>
  <cfset rtnVar = instance.json.decode(rtnVar)>

fixes the problem.