Unable ExportToStream HTML text

Hi. I am trying to export to stream HTML text with the following code:

var reportDocument = new ReportDocument();
reportDocument.Load(Server.MapPath(test_rpt file));
reportDocument.SetDataSource(test_source);
MemoryStream stream;
stream = (MemoryStream)this.reportDocument.ExportToStream(ExportFormatType.HTML32);
StreamReader reader = new StreamReader(stream);
string report = reader.ReadToEnd();
reader.Close();

But get just empty string!!!

But if I am using

reportDocument.ExportToDisk(ExportFormatType.HTML32, path_to_html)

All is working fine! Please help me to understand what is wrong?