Error: The required anti-forgery form field __RequestVerificationToken is not present in ASP.Net MVC

I'm getting an error while trying to load a modal window loading a different page. I was following this tutorial Implement and display jQuery Modal Popup Dialog (Window) in ASP.Net MVC Razor

However, following the tutorial the error I'm getting is this     

The required anti-forgery form field "__RequestVerificationToken" is not present.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.Web.Mvc.HttpAntiForgeryException: The required anti-forgery form field "__RequestVerificationToken" is not present. Source Error:

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What's the cause of this?