10426 Transaction Refused - Order Total Is Invalid 1.3.7.1

I'm operating on a brand new fresh install of 1.3.7.1 with only my template and the Easy Populate mod installed. I've followed the Paypal Wiki FAQ to the letter, but trying to check out any products results in the 10426 error.

This occurs regardless of my status as being logged in or not, and occurs regardless of free shipping being enabled for Australia only (my preference) or for everywhere. It's currently on everywhere, so feel free to try checking out some products. I've also tried with flat rate shipping set to $0, but the same error occurs.

The emailed error log:

2007-07-31 03:02:49
-------------------
(live transaction) --> "https://api-3t.paypal.com/nvp"
Request Headers: 
Array
(
    [0] => Content-Type: text/namevalue
    [1] => X-VPS-Timeout: 45
    [2] => X-VPS-VIT-Client-Type: PHP/cURL
    [3] => X-VPS-VIT-Integration-Product: PHP::Zen Cart - WPP-NVP
    [4] => X-VPS-VIT-Integration-Version: 0.1
)


Request Parameters: {SetExpressCheckout} 
Array
(
    [LOCALECODE] => AU
    [PAYMENTACTION] => Sale
    [EMAIL] => *******
    [PHONENUM] => 040299999613
    [AMT] => 0.00
    [RETURNURL] => MYSITE/ipn_main_handler.php?type=ec
    [CANCELURL] =>MYSITE/index.php?main_page=checkout_shipping&ec_cancel=1
    [PAGESTYLE] => Primary
    [METHOD] => SetExpressCheckout
    [CURRENCYCODE] => USD
    [USER] => s****_api1.headsonic.com
    [PWD] => ****************
    [VERSION] => 2.3
    [SIGNATURE] => ****************************************************vL7t
)


Response: 
Array
(
    [TIMESTAMP] => 2007-07-31T04:49:09Z
    [CORRELATIONID] => 6e5f1d57a071
    [ACK] => Failure
    [L_ERRORCODE0] => 10426
    [L_SHORTMESSAGE0] => Transaction refused because of an invalid argument.
 See additional error messages for details.
    [L_LONGMESSAGE0] => Order total is invalid.
    [L_SEVERITYCODE0] => Error
    [VERSION] => 2.300000
    [BUILD] => 1.0006
    [CURL_ERRORS] => 
)

Once again, this is a brand new fresh install of 1.3.7.1.

Thanks for the help!

What is your site default currency?
What other currencies are defined?
What are their exchange rates?

The site's default currency is the Australian dollar. The value is set to 1. The rest of the currencies have been removed from the cart.

Checking now, I am an absolute fool and I apologise for wasting your time. The express checkout was set to 'only USD'.

Sorry once again. I am going to donate out of sympathy.

Thank you for double checking your settings and reporting that you found the problem of having mis-matched currencies.