how to insert null in datetime

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C#

SQL-Server

SQL-Server-2008

hi,
i want to insert null value in datetime. i am using following code but it insert as 1900-01-01 00:00:00.000

con.Open();
string str="insert into table(podate) values(@PO_Date)";
cmd = new SqlCommand(str, con);
cmd.Parameters.Add(new SqlParameter("@PO_Date", SqlDbType.DateTime));
                sqldatenull = SqlDateTime.Null;
                if (PO_Date == "")
                {
                    cmd.Parameters["@PO_Date"].Value = System.Data.SqlTypes.SqlDateTime.Null;
                    //cmd.Parameters["@PO_Date"].Value = DBNull.Value;
                    //cmd.Parameters["@PO_Date"].Value = null;
                }
                else
                {
                    cmd.Parameters["@PO_Date"].Value = DateTime.Parse(PO_Date);
                }

podate insert as:1900-01-01 00:00:00.000;

pls help me.

thanking you in advance!!