ERD diagram for all entities failing with - Error: Found an attribute with unrecognized required level.

{System.ServiceModel.FaultException`1[Microsoft.Xrm.Sdk.OrganizationServiceFault]: Found an attribute with unrecognised required level.
(Fault Detail is equal to Microsoft.Xrm.Sdk.OrganizationServiceFault).}

       // Load the metadata.
       Console.WriteLine("Loading Metadata...");
       RetrieveAllEntitiesRequest request = new RetrieveAllEntitiesRequest()
       {
             EntityFilters = EntityFilters.Entity | EntityFilters.Attributes | EntityFilters.Relationships,
             RetrieveAsIfPublished = true,

      };
      RetrieveAllEntitiesResponse response = null;
      try
      {

     Error Point>   response = (RetrieveAllEntitiesResponse)_serviceProxy.Execute(request);
                    builder._metadataResponse = response;
      }
      catch (Exception ex)
      {

      }