Cannot set Column 'EffectiveDate' to be null. Please use DBNull instead.

dt.Columns.Add("EffectiveDate", typeof(DateTime));

   DateTime?   effectivedate=null;


  if (lastRow["Effective_x0020_Date"] != DBNull.Value)
  {

     effectivedate = Convert.ToDateTime(lastRow["Effective_x0020_Date"]);
  }


DataRow dr = dt.NewRow();
     dr["EffectiveDate"] = effectivedate;
dt.Rows.Add(dr);