title: Error with Entity Framework add-migration: "value cannot be null"
body: <p>i am working on an mvc4 asp.net web application. I wanted to add a migration but an error shows on and I don't know how to resolve it.
  Here's the code of the class:</p>
  
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Text;
  using System.Threading.Tasks;
  using System.Web;
  namespace POCOs
  {
  public class Consultant
  {
  public int Id { get; set; }
  public string Nom { get; set; }
  public string Prenom { get; set; }
  public string RefCommercial { get; set; }
  public string DateNaissance { get; set; }
  public int CIN { get; set; }
  public string Fonction { get; set; }
  public string Tel { get; set; }
  public string Experience { get; set; }
  public string Email { get; set; }
  public HttpPostedFileBase CV { get; set; }
  public ICollection&lt;Prestation&gt; prestations { get; set; }
  
  public Consultant()
  {
  prestations = new HashSet&lt;Prestation&gt;();
  }
  }
  }
  </code></pre>
  
  <p>and here's the error I get when I want to add a migration:  </p>
  
  <pre><code>Add-Migration
  cmdlet Add-Migration at command pipeline position 1
  Supply values for the following parameters:
  Name: cv
  System.ArgumentNullException: Value cannot be null.
  Parameter name: key
  at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
  at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue&amp; value)
  at System.Data.Entity.ModelConfiguration.Configuration.Mapping.SortedEntityTypeIndex.Add(EdmEntitySet entitySet, EdmEntityType entityType)
  at System.Data.Entity.ModelConfiguration.Configuration.Mapping.EntityMappingService.Analyze()
  at System.Data.Entity.ModelConfiguration.Configuration.ModelConfiguration.ConfigureEntityTypes(DbDatabaseMapping databaseMapping, DbProviderManifest providerManifest)
  at System.Data.Entity.ModelConfiguration.Configuration.ModelConfiguration.Configure(DbDatabaseMapping databaseMapping, DbProviderManifest providerManifest)
  at System.Data.Entity.DbModelBuilder.Build(DbProviderManifest providerManifest, DbProviderInfo providerInfo)
  at System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection)
  at System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext)
  at System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input)
  at System.Data.Entity.Internal.LazyInternalContext.InitializeContext()
  at System.Data.Entity.Internal.LazyInternalContext.get_CodeFirstModel()
  at System.Data.Entity.Infrastructure.EdmxWriter.WriteEdmx(DbContext context, XmlWriter writer)
  at System.Data.Entity.Migrations.Extensions.DbContextExtensions.&lt;&gt;c__DisplayClass1.&lt;GetModel&gt;b__0(XmlWriter w)
  at System.Data.Entity.Migrations.Extensions.DbContextExtensions.GetModel(Action`1 writeXml)
  at System.Data.Entity.Migrations.Extensions.DbContextExtensions.GetModel(DbContext context)
  at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext)
  at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration)
  at System.Data.Entity.Migrations.Design.ToolingFacade.BaseRunner.GetMigrator()
  at System.Data.Entity.Migrations.Design.ToolingFacade.GetPendingMigrationsRunner.RunCore()
  at System.Data.Entity.Migrations.Design.ToolingFacade.BaseRunner.Run()
  Value cannot be null.
  Parameter name: key 
  </code></pre>
  
  <p>Any help?</p>