Entity framework code first error: OriginalValues cannot be used for entities in the Added state

When I was using Entity framework code first, I encountered an error when I tried to create an entity into database.

The entity is:
[Table("EmployeeProfile")]
    public partial class EmployeeProfile
    {
        [Key]
        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
        public int EmployeeProfileID { get; set; }

        [ForeignKey("Employee")]
        public int EmployeeID { get; set; }
        public virtual Employee Employee { get; set; }

        [ForeignKey("Profile")]
        public int ProfileID { get; set; }
        public virtual Profile Profile { get; set; }

        [Required]
        [StringLength(255)]
        public string ProfileValue { get; set; }
    }

When creating the entity, some entities have the ProfileValue="", this causes the EntityValidationException with the detailed message "OriginalValues cannot be used for entities in the Added state". I want to allow the ProfileValue to be empty but also make it a require field. I found out that the Required attribute has a boolean parameter called "AllowEmptyStrings". Set this value to true will enable entity framework to create EmployeeProfile entity with empty string ProfileValue.