The code looks fine but does not run???

from urllib2 import urlopen
from json import load, dumps

url = 'http://api.npr.org/query?apiKey=' 
key = 'API_KEY'
url = url + key
url += '&numResults=1&format=json&id=1007' #1007 is science
url += '&requiredAssets=audio,text,image'
response = urlopen(url)
json_obj = load(response)


for story in json_obj['list']['story']:
print "TITLE: " + story['title']['$text'] + "\n"

I get a error with this message below: Did you create a query string parameter for requiredAssets?

Even though I have created requiredAssets with the appropirate atrributes, it does not run.

Need help