间歇性oledbException（0x80004005）：SQL0401：比较运算符=操作数不兼容

Question

我偶尔会在日志中看到这个例外：

12:27:38,504 [8] DEBUG - Inside GetRoles USER1
    12:27:38,537 [8] ERROR - Error in method, "GetRoles" . User account that invoked error-  USER1 
    System.Data.OleDb.OleDbException (0x80004005): SQL0401: Comparison operator = operands not compatible.
    Cause . . . . . :   The operands of comparison operator = are not compatible. -- Numeric operands are compatible with any other numeric operands and with character and graphic operands. -- Character operands are compatible with operands that are character, graphic, date, time, timestamp, or numeric. -- Date, time, and timestamp operands are compatible with character and graphic operands or with another operand of the same type. -- Graphic operands are compatible with graphic, character, date, time, timestamp, or numeric operands. -- Binary operands are compatible only with binary operands. -- Operands that are user-defined types can only be compared to operands that are the same exact type. -- DataLink and XML operands cannot be compared. Recovery  . . . :   Check the data types of all operands to see if the data types are compatible.  If all the operands of the SQL statement are correct and a view is being accessed, then check the data types of all the operands in the view definition. Correct the errors.  Try the request again.
       at System.Data.OleDb.OleDbCommand.ExecuteCommandTextErrorHandling(OleDbHResult hr)
       at System.Data.OleDb.OleDbCommand.ExecuteCommandTextForMultpleResults(tagDBPARAMS dbParams, Object& executeResult)
       at System.Data.OleDb.OleDbCommand.ExecuteCommandText(Object& executeResult)
       at System.Data.OleDb.OleDbCommand.ExecuteReaderInternal(CommandBehavior behavior, String method)
       at System.Data.OleDb.OleDbCommand.ExecuteReader(CommandBehavior behavior)
       at Dashboard.DataAccessLayer.DAL.GetRoles() in DAL.cs:line 98

这是我的代码：

public List<string> GetRoles()
        {
            List<string> tempList = new List<string>();
            OleDbConnection connection;
            OleDbCommand command;
            string sql = null;

            sql = "SELECT ROLE FROM " + LibraryList.PROJ + ".ROLE where UPPER(USERID) = UPPER(?)";
            using (connection = new OleDbConnection(_connectionString))
            {
                using (command = new OleDbCommand())
                {
                    try
                    {
                        connection.Open();
                        command.Connection = connection;
                        command.Parameters.Add("@userID", OleDbType.Char, 10).Value = CurrentUser.getUserID();
                        log.Debug("Inside GetRoles " + CurrentUser.getUserID()); 
                        command.CommandText = sql;

                        using (OleDbDataReader reader = command.ExecuteReader())
                        {

                            while (reader.Read())
                            {
                                tempList.Add(reader.GetString(0));
                            }
                        }
                    }
                    catch (Exception ex)
                    {
                        log.Error("Error in method, \"GetRoles\" . User account that invoked error- " + CurrentUser.getCurrentUserDisplayInfoForLogging(), ex);
                    }
                }
            }
            return tempList;
        }

我打印出值，它是一个字符串，所以它不是null或空或任何东西。这种情况很少发生，因此当看起来价值与其他任何时间相同时，很难确定发生了什么，但有时却不喜欢它。

可能导致此问题的原因是什么？