Problem With Null Concurrency Checks On Date Field

I am having a problem building a sqlDataAdapter in code which correctly tests for a null date in the concurrency check. The below method builds the command object I need. All the null tests work correctly except for the date. Anyone have any ideas why the date fails the check?

private SqlCommand adContactListUpdateCommand()
{
      // Update Logic
      string sql = "UPDATE Contact " +
        "SET FirstName = @FirstName_New, " +
          "MiddleName = @MiddleName_New, " +
          "LastName = @LastName_New, " +
          "BirthDate = @BirthDate_New, " +
          "FamilyID = @FamilyID_New, " +
          "BusinessID = @BusinessID_New, " +
          "GroupID = @GroupID_New " +
        "WHERE ContactID = @ContactID " +
          "AND FirstName = @FirstName_Old " +
          "AND MiddleName = @MiddleName_Old " +
          "AND LastName = @LastName_Old " +
          "AND ( ( @BirthDate_Old IS NULL AND BirthDate IS NULL ) OR BirthDate = @BirthDate_Old ) " +
          "AND ( ( FamilyID IS NULL AND @FamilyID_Old IS NULL ) OR FamilyID = @FamilyID_Old ) " +
          "AND ( ( BusinessID IS NULL AND @BusinessID_Old IS NULL ) OR BusinessID = @BusinessID_Old ) " +
          "AND ( ( GroupID IS NULL AND @GroupID_Old IS NULL ) OR GroupID = @GroupID_Old )";
           
      // Create Command
      SqlCommand cmd = new SqlCommand(sql, sqlCon);

      // Add New Parameters
      cmd.Parameters.Add("@FirstName_New", SqlDbType.NVarChar, 0, "FirstName");
      cmd.Parameters.Add("@MiddleName_New", SqlDbType.NVarChar, 0, "MiddleName");
      cmd.Parameters.Add("@LastName_New", SqlDbType.NVarChar, 0, "LastName");
      cmd.Parameters.Add("@BirthDate_New", SqlDbType.DateTime, 0, "BirthDate");
      cmd.Parameters.Add("@FamilyID_New", SqlDbType.Int, 0, "FamilyID");
      cmd.Parameters.Add("@BusinessID_New", SqlDbType.Int, 0, "BusinessID");
      cmd.Parameters.Add("@GroupID_New", SqlDbType.Int, 0, "GroupID");

      // Primary Key Parameters
      cmd.Parameters.Add("@ContactID", SqlDbType.Int, 0, "ContactID");

      // Add Old Parameters
      SqlParameter par = new SqlParameter();
      par = cmd.Parameters.Add("@FirstName_Old", SqlDbType.NVarChar, 0, "FirstName");
      par.SourceVersion = DataRowVersion.Original;
      par = cmd.Parameters.Add("@MiddleName_Old", SqlDbType.NVarChar, 0, "MiddleName");
      par.SourceVersion = DataRowVersion.Original;
      par = cmd.Parameters.Add("@LastName_Old", SqlDbType.NVarChar, 0, "LastName");
      par.SourceVersion = DataRowVersion.Original;
      par = cmd.Parameters.Add("@BirthDate_Old", SqlDbType.DateTime, 0, "BirthDate");
      par.SourceVersion = DataRowVersion.Original;  
      par.SourceColumnNullMapping = true;
      par = cmd.Parameters.Add("@FamilyID_Old", SqlDbType.Int, 0, "FamilyID");
      par.SourceVersion = DataRowVersion.Original;
      par = cmd.Parameters.Add("@BusinessID_Old", SqlDbType.Int, 0, "BusinessID");
      par.SourceVersion = DataRowVersion.Original;
      par = cmd.Parameters.Add("@GroupID_Old", SqlDbType.Int, 0, "GroupID");
      par.SourceVersion = DataRowVersion.Original;

      // Return Command
      return cmd;
}