WWW.Addbinarydata error 417

I am trying to send some data to the server using www class.

I am using the following code as also given in http://docs.unity3d.com/ScriptReference/WWWForm.html and given below:

Code (CSharp):

1.  
2.     IEnumerator UploadPNG() {
3.         // We should only read the screen after all rendering is complete
4.         yield return new WaitForEndOfFrame();
5.  
6.         // Create a texture the size of the screen, RGB24 format
7.         int width = Screen.width;
8.         int height = Screen.height;
9.         var tex = new Texture2D( width, height, TextureFormat.RGB24, false );
10.  
11.         // Read screen contents into the texture
12.         tex.ReadPixels( new Rect(0, 0, width, height), 0, 0 );
13.         tex.Apply();
14.  
15.         // Encode texture into PNG
16.         byte[] bytes = tex.EncodeToPNG();
17.         Destroy( tex );
18.  
19.         // Create a Web Form
20.         WWWForm form = new WWWForm();
21.         form.AddField("frameCount", Time.frameCount.ToString());
22.         form.AddBinaryData("fileUpload", bytes, "screenShot.png", "image/png");
23.  
24.         // Upload to a cgi script
25.         WWW w = new WWW(screenShotURL, form);
26.         yield return w;
27.         if (!string.IsNullOrEmpty(w.error)) {
28.             print(w.error);
29.         }
30.         else {
31.             print("Finished Uploading Screenshot");
32.         }
33.     }
34. }

I am getting an error as Error 417: Expectation Failure.

When I remove the line form.addbinarydata(), then everything works fine.

Any suggestion will be of great help.
Thanks