Sql EF 4.1 异常提供程序未返回 ProviderManifestToken 字符串

我试图复制MSDN上的一个例子.我正在使用ASP.NET和EF 4.1(CTP？).我已经使用NuGet安装了EntityFramework软件包.

我得到了这个错误:The provider did not return a ProviderManifestToken string...

这是我的连接字符串:

<add name="HospitalContext"
   connectionString=
   "data source=.\SQLExpress;initial catalog=NewTestDB;integrated security=True;"
   providerName="System.Data.SqlClient"/>

这是我的代码:

var pat = new Patient { Name = "Shane123132524356436435234" };
db.Patients.Add(pat);

var labResult = new LabResult { Result = "bad", Patient = pat };

int recordAffected = db.SaveChanges();

以下是我的背景:

public class HospitalContext : DbContext
{
    static HospitalContext()
    {
        Database.SetInitializer(new HostpitalContextInitializer());
    }

    public DbSet<Patient> Patients { get; set; }
    public DbSet<LabResult> LabResults { get; set; }
}

public class HostpitalContextInitializer :
             DropCreateDatabaseIfModelChanges<HospitalContext>
{
    protected override void Seed(HospitalContext context)
    {
        context.Patients.Add(new Patient { Name = "Fred Peters" });
        context.Patients.Add(new Patient { Name = "John Smith" });
        context.Patients.Add(new Patient { Name = "Karen Fredricks" });
    }
}

这是一个完全修补的SQL 2008系统，带有VS 2010 SP1.