"Membership.Provider must be an instance of ExtendedMembershipProvider"

I'm writing a MVC4 Website using Microsoft Visual Studio express 2012 for web. Whenever I run "Update-Database" in the Package Manager Console, the following Exception occurs:

To call this method, the "Membership.Provider" property must be an instance
of "ExtendedMembershipProvider".

This is my Seed method:

protected override void Seed(GNSystem.Models.DataContext context)
    {
        context.Forums.AddOrUpdate(
                        new Forum { ForumName = "Hello" },
                        new Forum { ForumName = "World" },
                        new Forum { ForumName = "!" }
                        );
        context.UserProfiles.Add(new UserAccount { UserName = "Gilad", EMail = "user@example.com" });
        WebSecurity.CreateUserAndAccount("Gilad", "123456");
        WebSecurity.Login("Gilad", "123456");
        context.Threads.AddOrUpdate(
            new Thread { Subject = "FirstThread", Content = "Awesome Content", ForumID = 1, UserID = 1 },
            new Thread { Subject = "SecondThread", Content = "Awesome Content", ForumID = 1, UserID = 1 }
            );
    }

I searched for solutions and added the following directive in the Web.config file:

<add key="enableSimpleMembership" value="true" />

but the exception is thrown nonetheless.

What is the cause of this? Edit 1:

    <?xml version="1.0" encoding="utf-8"?>
<!--
  For more information on how to configure your ASP.NET application, please visit
  http://go.microsoft.com/fwlink/?LinkId=169433
  -->
<configuration>
  <configSections>
    <!-- For mor