addpaymentcard trouble

addpaymentcardnull.py

def set_card():
    test_email='eli_1335983746_per@simplerelevance.com'

    headers = {"X-PAYPAL-SECURITY-USERID":PAYPAL_API_USERNAME,"X-PAYPAL-SECURITY-PASSWORD":PAYPAL_API_PASSWORD,"X-PAYPAL-SECURITY-SIGNATURE":PAYPAL_API_SIGNATURE,"X-PAYPAL-APPLICATION-ID":"APP-80W284485P519543T","X-PAYPAL-REQUEST-DATA-FORMAT":"NV","X-PAYPAL-RESPONSE-DATA-FORMAT":"JSON"}

    payload = {
        'requestEnvelope.errorLanguage':'en_US',
        'emailAddress':test_email,
        'cardNumber':'5178054152614319',
        'cardType':'MasterCard',
        'confirmationType':'WEB',
        'nameOnCard.firstName':'john',
        'nameOnCard.lastName':'smith',
        'billingAddress.line1':'1443 chicago ave',
        'billingAddress.city':'Chicago',
        'billingAddress.state':'IL',
        'billingAddress.postalCode':'60660',
        'billingAddress.countryCode':'US',
        'expirationDate.year':'2014',
        'expirationDate.month':'6',

}
    payload = urllib.urlencode(payload)
    print payload

    resp = requests.post('https://svcs.sandbox.paypal.com/AdaptiveAccounts/AddPaymentCard',headers=headers,data=payload)
    print resp.status_code
    print resp.content

response is:

{"responseEnvelope":{"timestamp":"2012-05-10T07:47:32.802-07:00","ack":"Failure","correlationId":"2ab6eaaa830c4","build":"2810423"},"error":[{"errorId":"580029","domain":"PLATFORM","subdomain":"Application","severity":"Error","category":"Application","message":"Missing required request parameter: null","parameter":["null"]}]}