New Post: Update-Database throws NullReferenceException (alpha 3)

I am using EF 5 on a .net 4.0 WPF application (so it is 4.4.0.0) - I decided to remove it and install 6 alpha 3, removed all references of System.Data.Entity.dll, added EF 6 alpha-3 using nuget, restarted VS, fixed some problems, all compiled and fine. Just to be sure, I dropped my old test database and tried the command:
Update-Database
The results:

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Data.Entity.Edm.Serialization.EdmSerializationVisitor.VisitEdmAssociationSet(AssociationSet item)
   at System.Data.Entity.Edm.EdmModelVisitor.VisitCollection[T](IEnumerable`1 collection, Action`1 visitMethod)
   at System.Data.Entity.Edm.EdmModelVisitor.VisitAssociationSets(EntityContainer container, IEnumerable`1 associationSets)
   at System.Data.Entity.Edm.EdmModelVisitor.VisitEdmEntityContainer(EntityContainer item)
   at System.Data.Entity.Edm.Serialization.EdmSerializationVisitor.VisitEdmEntityContainer(EntityContainer item)
   at System.Data.Entity.Edm.EdmModelVisitor.VisitCollection[T](IEnumerable`1 collection, Action`1 visitMethod)
   at System.Data.Entity.Edm.EdmModelVisitor.VisitEntityContainers(IEnumerable`1 entityContainers)
   at System.Data.Entity.Edm.EdmModelVisitor.VisitEdmModel(EdmModel item)
   at System.Data.Entity.Edm.Serialization.EdmSerializationVisitor.Visit(EdmModel edmModel, String provider, String providerManifestToken)
   at System.Data.Entity.Edm.Serialization.SsdlSerializer.Serialize(EdmModel dbDatabase, String provider, String providerManifestToken, XmlWriter xmlWriter, Boolean serializeDefaultNullability)
   at System.Data.Entity.ModelConfiguration.Edm.Serialization.EdmxSerializer.WriteEdmxRuntime()
   at System.Data.Entity.ModelConfiguration.Edm.Serialization.EdmxSerializer.Serialize(DbDatabaseMapping databaseMapping, DbProviderInfo providerInfo, XmlWriter xmlWriter)
   at System.Data.Entity.Infrastructure.EdmxWriter.WriteEdmx(DbModel model, XmlWriter writer)
   at System.Data.Entity.Infrastructure.EdmxWriter.WriteEdmx(DbContext context, XmlWriter writer)
   at System.Data.Entity.Utilities.DbContextExtensions.<>c__DisplayClass1.<GetModel>b__0(XmlWriter w)
   at System.Data.Entity.Utilities.DbContextExtensions.GetModel(Action`1 writeXml)
   at System.Data.Entity.Utilities.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.UpdateRunner.RunCore()
   at System.Data.Entity.Migrations.Design.ToolingFacade.BaseRunner.Run()
Object reference not set to an instance of an object.

Any hint on where can I look for the problem?