Created Unassigned: Encoding problems [9478]

Hi,
I create a worksheet stored in MemoryStream then read as a string. I have to pass this string as base64 encoded via ajax reqest.
The problem is that i get back something like this "PK[QHó·f ¸xl/workbook.xml ˘( ŤMnÂ0…ŻbÍľ8D˘­"›nŘT•ŠÚµ±ÇÄ"¶#ŹÜ­‹©W¨ˆ@]uĺůűŢĽńĎ×÷r=¸Žť0’...."
I tried with default encoding and utf-8 encoding but not worked!
This is my code :
public string DataTableToXLS(DataTable sourceTable)
{

Stream fs = new MemoryStream();
var wb = new XLWorkbook();
wb.Worksheets.Add(sourceTable);
wb.SaveAs(fs);
fs.Position = 0;
var sr = new StreamReader(fs,Encoding.Default,false);
var myStr = sr.ReadToEnd();
return myStr;
}