AjaxFileUpload Issue

I get the following error:

Error: Sys.ArgumentException: Cannot deserialize. The data does not correspond to valid JSON.
Parameter name: data
ScriptResource.axd:16:15
POST
XHR
http://localhost:49500/AjaxFileUploadHandler.axd [HTTP/1.1 200 OK 96ms]
no element foundAjaxFileUploadHandler.axd:1:1
POST
XHR
http://localhost:49500/Inventory_List.aspx [HTTP/1.1 200 OK 123ms]
Error: Sys.ArgumentException: Cannot deserialize. The data does not correspond to valid JSON.
Parameter name: data

I have successfully implemented the Ajaxfileupload on another web form within my website and I don't understand why it doesn't work within this web form.

I believe the only difference between the Web forms is that the one that doesn't work actually has a query string.

Any help would be greatly appreciated