EntityType has no key defined. Define the key for this EntityType

Could someone help with this error for the class below: Error: GEW.Models.Repository.Product: : EntityType 'Product' has no key defined. Define the key for this EntityType. ========================== Product.cs ========================== namespace GEW.Models {     public class Product {         public int pkProductID { get; set; }         public string Description { get; set; }         public decimal Cost { get; set; }     } }