In Microsoft SharePoint Designer 2010, you use the External Content Type designer to generate methods on an external content type. However, when you use the GenerateAll option in the External Content Type designer, the wrong type name is assigned to the TypeDescriptors. These TypeDescriptors are marked as identifiers, and they have a nullable type. In this situation, the type names become non-nullable types.
The mapping of TypeDescriptors (whose type is nullable) to identifiers is not supported in Business Connectivity Services.
Microsoft has confirmed that this is a problem in SharePoint Designer 2010.