Not Saving Entries on EntityFramework

Question

when i add item my HomeController.cs shows this error pointing in db.SaveChanges(); following this tutorial

 

http://kristianguevara.net/creating-your-asp-net-mvc-5-application-from-scratch-for-beginners-using-entity-framework-6-and-identity-with-crud-functionalities/#comment-4559 

 

An exception of type 'System.Data.Entity.Infrastructure.DbUpdateException' occurred in EntityFramework.dll but was not handled in user code

Additional information: An error occurred while updating the entries. See the inner exception for details.