how to save a null date value to oracle database

public static void AddNewStudent(int Stu_id, string Stu_Name, string Class, string Major, Datetime Enddate)
  {
  OracleDatabase db =(OracleDatabase)DatabaseFactory.CreateDatabase("PHTS");
 using (System.Data.Common.DbCommand cmd = db.GetStoredProcCommand("AddNewStudent"))  
           {
  db.AddInParameter(cmd, "I_Stu_id", DbType.Int32, Stu_id);
  db.AddInParameter(cmd, "I_Stu_Name", DbType.String, Stu_Name);
  db.AddInParameter(cmd, "I_Class", DbType.String, Class);
  db.AddInParameter(cmd, "I_Major", DbType.String, Major);
  db.AddInParameter(cmd, "I_Enddate", DbType.Date, Enddate, Enddate == null? DBNull.Value : Enddate);
    db.ExecuteNonQuery(cmd);
            }
  }

private void Savebutton_Click(object sender, EventArgs e)
{
private void Savebutton_Click(object sender, EventArgs e)
        {
            int student_id = Convert.ToInt32(this.SIDtextBox.Text);
            string stu_name = this.textBox1.Text;
            string class = this.textBox2.Text;
            string major= this.textBox3.Text;
	    Datetime enddate = Datetime.Parse(this.textbox4.text);
 if (this.textbox4.Text == "")
            {
                enddate = null;// DBNull.Value;
            }
            else
            {
                enddate = DateTime.Parse(this.textbox4.Text);
            }

 
PHTS.DataServices.cslMaterial.AddNewStudent(stu_id, stu_name, class, major, enddate);
            MessageBox.Show("Record Saved");

}
}
}

Create or replace PRPCEDURE AddNewStudent (
I_STU_ID    Number,    
I_STU_Name  Varchar2,
I_Address  Varchar2,
I_Phone  Varchar2,
I_Enddate   date,
)
As
Begin
Insert into I_Student
(
Student_ID, STU_Name, Address, Phone, Enddate
)
Values
(
I_Student_ID, I_STU_Name, I_Address, I_phone, I_Date , I_Enddate
);