Differenza DateTime in Entity Framework Core

Ho la seguente query su Entity Framework Core:

public class Question {
  public DateTime? Approved { get; set; }
  public DateTime Created { get; set; }
}

public class QuestionModel {
  public TimeSpan ResponseTime { get; set; }
}

List<Question> questions = await context.Questions
  .Select(x =>
    new QuestionModel {
      ResponseTime = x.Approved.Value - x.Created
    }).ToListAsync();

Ma ottengo il seguente errore:

System.Data.SqlClient.SqlException: Operand data type datetime2 is invalid for subtract operator.

Come posso ottenere una differenza DateTime con Entity Framework Core?