Entity Type Has No Key Defined

Another 'Entity Type 'x' has no key defined' question, but I've set the `[Key]` attribute on a property so I'm a bit confused. Here's my entity and context classes: ``` namespace DoctorDB.Models { public class Doctor { [Key] public string GMCNumber; [Required] public string givenName; [Required] public string familyName; public string MDUNumber; public DateTime MDUExpiry; public string MDUCover; } public class DoctorContext : DbContext { public DbSet<Doctor> Doctors { get; set; } } } ``` When I go to create my controller, I've selected to create it with the Entity Framework methods using this entity and context: and I get this error: My only thought is whether you can't successfully use [Key] on a string property. If you can't then fair enough, I'll work round it, but I'd be grateful if someone could confirm this one way or the other.