值不能为空。参数名称:在ASP.NET中进行数据绑定时的键

我正在尝试将数据绑定到来自sql server的列表框,然后得到错误“值不能为空。参数名称:key”

ddlCountry = new Obout.ComboBox.ComboBox();
ddlCountry.Width = 200;
ddlCountry.Height = 200;
ddlCountry.DataTextField = "Country";
ddlCountry.DataValueField = "Country";

sqlCommand = "SELECT [Country] FROM [tbl_LookupCountry] where [Country] IS NOT NULL";
SqlConnection sqlConCountry = new SqlConnection(connectString);
SqlCommand sqlCommCountry = new SqlCommand();
sqlCommCountry.Connection = sqlConCountry;
sqlCommCountry.CommandType = System.Data.CommandType.Text;
sqlCommCountry.CommandText = sqlCommand;
sqlCommCountry.CommandTimeout = 300;
sqlConCountry.Open();
reader = sqlCommCountry.ExecuteReader();
ddlCountry.DataSource = reader;
ddlCountry.DataBind();
sqlConCountry.Close();

以前有人遇到过这个问题吗?