Scrapy FormRequest sending JSON

I am trying to create a FormRequest that can send content-type:application/json.

Here is what I try:

yield FormRequest("abc.someurl.com", formdata=json.dumps({"referenceId":123,"referenceType":456}), headers={'content-type':'application/json'}, callback=self.parseResult2)

If I use json.dumps() to process the form data in the formdata=, the error I get is

"exceptions.ValueError: need more than 1 value to unpack"

I can't just use the list as in

formdata={"referenceId":123,"referenceType":456} 

The FormRequest works but is not accepted by the server.

import requests
import json
result = requests.post(url, json.dumps({"referenceId":123,"referenceType":456}), headers={'content-type':'application/json'})

It works from the python command prompt as in the above.

Any ideas?

-KM