Cause
This is an issue with jquery.validate.unobtrusive.js
in your ASP.NET.MVC package.
As of jQuery 1.9, the behavior of parseJSON()
has changed and an undefined
value would be considered a malformed JSON, resulting in the error you've specified. See the jQuery 1.9 Core Upgrade Guide for more information.
Solution
Use the jQuery Migrate plugin, which among other things adds backward-compatibility to the jQuery parseJSON()
utility.
EDIT
According to the official announcement in this thread on Microsoft Connect, the issue has been resolved in the latest release of the framework.
Naturally, as Andreas Larsen noted in the comments, make sure to clear any relevant cache, server-side and client-side, after upgrading to the new release.