Entity Framework throwing Null entry error at Saving, C#

I am working on Entity Framework 6 and repositories setup to do crud operations. I am trying to insert record in one of the table and getting error for null entries even duo it is not.

{"Cannot insert the value NULL into column 'UI_ID', table 'Blackpool_BPM.dbo.COURSE_INSTANCE'; column does not allow nulls. INSERT fails.\r\nThe statement has been terminated."}

The connection to database is correct as I can read data with no problem

At Generic Repository, getting data correctly, just before Save()

Table Structure

Class Model

 [Table("COURSE_INSTANCE")]
public class BIZCourseInstanceEntity
{
    [Key]
    public int UI_ID { get; set; }

    public string UnitInstanceCode { get; set; }

    public string FESLongDescription { get; set; }


    public string FESShortDescription { get; set; }

    public string FullDescription { get; set; }

    public string OwningOrganisationCode { get; set; }

    public int? OwningOrganisationID { get; set; }

    public string TopicCode { get; set; }


    public string UnitCategory { get; set; }

    public string UnitCode { get; set; }

    public string FESQualificationType { get; set; }

    public int? SCHOOLS { get; set; }

    public int? MARKETING_GROUPS { get; set; }

}

Repository

 public class BIZCourseInstanceRepository : GenericRepository
{
    public BIZCourseInstanceRepository() { }

    public BIZCourseInstanceRepository(DbContext dbContext)

        :base(dbContext)
    { }
}

Unit of work class

  public class BIZ_UOF : IDisposable
{
    private BIZDbContext _BIZDbContextObject = new BIZDbContext();


    protected BIZCourseInstanceRepository _BIZCourseInstanceRepository;

 public BIZCourseInstanceRepository BIZCourseInstanceRepository
    {
        get
        {
            if (this._BIZCourseInstanceRepository == null)
            {
                this._BIZCourseInstanceRepository = new BIZCourseInstanceRepository(_BIZDbContextObject);

            }

            return _BIZCourseInstanceRepository;
        }
    }
    /////

   public void Save()
    {
        _BIZDbContextObject.Database.Log = s => System.Diagnostics.Debug.WriteLine(s);


        _BIZDbContextObject.SaveChanges();
    }

    private bool disposed = false;

    protected virtual void Dispose(bool disposing)
    {
        if (!this.disposed)
        {

            if (disposing)
            {
                _BIZDbContextObject.Dispose();
            }
        }
        this.disposed = true;
    }

    public void Dispose()
    {

        Dispose(true);
        GC.SuppressFinalize(this);
    }

DbContext

public class BIZDbContext : BaseContext
{
    public BIZDbContext() : base("_DbContext")

    { }

    public DbSet BIZ_CourseInstance { get; set; }     
}

Generic Repository CRUD

 public void InsertEntity(TEntity obj)
    {

        _DbSet.Add(obj);
    }

Function class where Error is generating at Save()

  public void InsertCourseInstance()
    {
        BIZCourseInstanceEntity BIZCourseInstanceEntityObject = null;


        BIZCourseInstanceEntityObject = new BIZCourseInstanceEntity
        {
            UI_ID = 999999,
            UnitInstanceCode = "KZ999999",
            FESLongDescription = "LONG",
            FESShortDescription = "SHORT",
            FullDescription = "FULL",
            OwningOrganisationCode = "E",
            OwningOrganisationID = 155,
            TopicCode = "04.1",

            UnitCategory = "04",
            UnitCode = "HE-G", 
            FESQualificationType = null,
            SCHOOLS = 5,
            MARKETING_GROUPS = 44

        };

        using (var _uow = new BIZ_UOF())
        {

            _uow.BIZCourseInstanceRepository.InsertEntity(BIZCourseInstanceEntityObject);

            _uow.Save();
        }
    }

Answer

I have found answer, the problem was in my database I am require to provide the Primary key which in my case is UI_ID but in my model I haven't define DatabaseGeneredOption.None, hence throwing error, Thanks for Krillgar guiding me

here is updated model

[Table("COURSE_INSTANCE")]
public class BIZCourseInstanceEntity
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int UI_ID { get; set; }

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


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

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

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


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

    public int? OwningOrganisationID { get; set; }

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

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


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

    [StringLength(50)]
    public string FESQualificationType { get; set; }

    public int? SCHOOLS { get; set; }

    public int? MARKETING_GROUPS { get; set; }

}