PostgresException: 23505: il valore chiave duplicato viola il vincolo univoco "PK_country"

Sto usando EF Core 2.0 e Postgres 9.6. Ogni volta che inserisco i dati ottengo l'errore

PostgresException: 23505: il valore chiave duplicato viola il vincolo univoco "PK_country"

Tracciando sembra che EF non generi la colonna AutoIncrement

Il mio modello

public partial class Country
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int CountryId { get; set; }
        [Display(Name="Country Name")]
        public string CountryName { get; set; }
    }

Il mio controller

if (ModelState.IsValid)
            {
                _context.Add(country);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }

Eccezione

> PostgresException: 23505: duplicate key value violates unique
> constraint "PK_country"
> 
>     Npgsql.NpgsqlConnector+<DoReadMessage>d__148.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
> task)
>     System.Runtime.CompilerServices.TaskAwaiter.GetResult()
>     System.Runtime.CompilerServices.ValueTaskAwaiter.GetResult()
>     Npgsql.NpgsqlConnector+<ReadMessage>d__147.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     Npgsql.NpgsqlConnector+<ReadMessage>d__147.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
> task)
>     System.Runtime.CompilerServices.TaskAwaiter.GetResult()
>     System.Runtime.CompilerServices.ValueTaskAwaiter.GetResult()
>     Npgsql.NpgsqlDataReader+<NextResult>d__32.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
> task)
>     Npgsql.NpgsqlDataReader+<<NextResultAsync>b__31_0>d.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
> task)
>     Npgsql.NpgsqlCommand+<Execute>d__71.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
> task)
>     System.Runtime.CompilerServices.TaskAwaiter.GetResult()
>     System.Runtime.CompilerServices.ValueTaskAwaiter.GetResult()
>     Npgsql.NpgsqlCommand+<ExecuteDbDataReader>d__92.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
> task)
>     System.Runtime.CompilerServices.TaskAwaiter.GetResult()
>     System.Runtime.CompilerServices.ValueTaskAwaiter.GetResult()
>     Npgsql.NpgsqlCommand+<>c__DisplayClass90_0+<<ExecuteDbDataReaderAsync>b__0>d.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
> task)
>     System.Runtime.CompilerServices.TaskAwaiter.GetResult()
>     Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand+<ExecuteAsync>d__17.MoveNext()
>     System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
>     System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
> task)
>     System.Runtime.CompilerServices.TaskAwaiter.GetResult()
>     Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch+<ExecuteAsync>d__32.MoveNext()

Quale potrebbe essere il problema?