WCF REST Template and 500.19

In VS2010, using WCF REST template and running your web service in IIS,  you may  get the following error:

HTTP  Error 500.19 – Internal Server Error

The  requested page cannot be accessed because the related configuration data for  the page is invalid.

The error is misleading.

Fix it by making sure your site Application Pool is running  under ASP.NET 4.0.