问GET查询中列之间的类型不匹配。当只返回NULL时，如何定义返回列类型？

EN

DataTable dataTable = new DataTable
{
    Columns = 
    {
        new DataColumn("A", typeof(string)){ AllowDBNull = true },
        new DataColumn("B", typeof(string)){ AllowDBNull = true },
        new DataColumn("C", typeof(string)){ AllowDBNull = true },
        new DataColumn("D", typeof(string)){ AllowDBNull = true }
    }
} 

await using DbConnection connection = new SqliteConnection("Data Source=MyDatabase.db");
await using DbCommand command = connection.CreateCommand();
string selectCommand = $"SELECT NULL as 'A', NULL as 'B', C, D FROM Table ";
command.CommandText = selectCommand;

await connection.OpenAsync().ConfigureAwait(false);
await using DbDataReader reader = await command.ExecuteReaderAsync().ConfigureAwait(false);
dataTable.Load(reader);
await connection.CloseAsync().ConfigureAwait(false);

dataTable.Load(reader);

System.InvalidOperationException: 'Inconvertible type mismatch between SourceColumn 'A' of Byte[] and the DataColumn 'A' of String.'

string selectCommand = $"SELECT CAST(NULL as TEXT) as 'A', CAST(NULL as TEXT) as 'B', C, D FROM Table ";