خطای MultiStrings: EntityType: EntitySet 'MultiStrings' is based on type 'MultiString' that has no k

سلام..
من از code first استفاده میکنم.یکی از مدلهامو وقتی به فیلدهاش [Required(ErrorMessageResourceName = "Required", ErrorMessageResourceType = typeof(Resources))] رو نسبت می دم ، این خطا رو میده:


کسی میدونه دلیلش چیه؟

One or more validation errors were detected during model generation:

WebSite.Models.MultiString: : EntityType 'MultiString' has no key defined. Define the key for this EntityType.
MultiStrings: EntityType: EntitySet 'MultiStrings' is based on type 'MultiString' that has no keys defined.

روی پراپرتی ID کلاس فوق صفت [Key] اضافه کنید.