Uploading a pdf file to Document Set programmatically using FileUpload

I am trying to upload a pdf file to a Document set using following code, but it generates this error File contains corrupted data. when it comes to the last line of the code

ContentPlaceHolder cph = (ContentPlaceHolder)this.Master.FindControl("PlaceHolderMain");
FileUpload fileUpload = (FileUpload)cph.FindControl("AddPdfFileUpload");

if (fileUpload.HasFile)
{
    DocumentSet documentSet = DocumentSet.GetDocumentSet(ListItem.Folder);

    byte[] pdfBytes = fileUpload.FileBytes;


    SPContentTypeId contentTypeId = List.ContentTypes["TemplateCT"].Id;

    string documentSetName = ListItem.Name;
    SPFolder targetFolder = List.RootFolder;
    Hashtable properties = ListItem.Properties;
    DocumentSet newDocSet = DocumentSet.Import(pdfBytes, documentSetName, targetFolder, contentTypeId, properties, CurrentWeb.CurrentUser);

}

I need to retrieve the FileUpload using above code because it has been added programmatically. I have debugged the code and the pdfBytes contains data and isn't empty.