问如何在asp.net中修复“列'xxxx'不允许DBNull.Value。”

EN

protected void Upload_Click(object sender, EventArgs e)
{
    string excelPath = Server.MapPath("~/Nova pasta/") + Path.GetFileName(FileUpload1.PostedFile.FileName);

    string filepath = Server.MapPath("~/Nova pasta/") + Path.GetFileName(FileUpload1.FileName);
    string filename = Path.GetFileName(filepath);
    FileUpload1.SaveAs(excelPath);
    string ext = Path.GetExtension(filename);

    String strConnection = @"Data Source=PEDRO-PC\SQLEXPRESS;Initial Catalog=costumizado;Persist Security Info=True;User ID=sa;Password=1234";
    string excelConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + filepath + ";Extended Properties=\"Excel 12.0 Xml;HRD=YES;IMEX=1;\"";

    OleDbConnection excelConnection = new OleDbConnection(excelConnectionString);

    OleDbCommand cmd = new OleDbCommand("Select * from [rptListaMovs_4$]", excelConnection);
    excelConnection.Open();

    cmd.ExecuteNonQuery();

    DataSet ds = new DataSet();
    SqlDataAdapter da = new SqlDataAdapter("Select * from [rptListaMovs_4$]", strConnection);
    OleDbDataReader dReader;
    dReader = cmd.ExecuteReader();
    using (SqlBulkCopy sqlBulk = new SqlBulkCopy(strConnection))
    {
        sqlBulk.DestinationTableName = "Dados";
        sqlBulk.ColumnMappings.Add("Data Mov", "Data Mov.");
        sqlBulk.ColumnMappings.Add("Data Valor", "Data Valor");
        sqlBulk.ColumnMappings.Add("Descrição do Movimento", "Descrição do Movimento");
        sqlBulk.ColumnMappings.Add("Valor em EUR", "Valor em EUR");

        sqlBulk.WriteToServer(dReader);
    }
    excelConnection.Close();
}

sqlBulk.WriteToServer(dReader);

CREATE TABLE [dbo].[Dados] 
(
    [Data Mov.] [VARCHAR](200) NULL,
    [Data Valor] [VARCHAR](200) NULL,
    [Descrição do Movimento] [VARCHAR](200) UNIQUE NOT NULL,
    [Valor em EUR] [VARCHAR](200) NULL
)