PayPal Checkout Error

I'm still developing and testing my store, and I just entered the necessary paypal business account info in Modules >>> Payment. Now, in testing mode, when I add a product to my cart and click Check Out With PayPayl, I get the following error:

Warning: array_merge() [function.array-merge]: Argument #1 is not an array in /home/jacksold/public_html/store/includes/modules/payment/paypalwpp.php on line 1467

Warning: array_merge() [function.array-merge]: Argument #2 is not an array in /home/jacksold/public_html/store/includes/modules/payment/paypalwpp.php on line 1467

Warning: Cannot modify header information - headers already sent by (output started at /home/jacksold/public_html/store/includes/modules/payment/paypalwpp.php:1467) in /home/jacksold/public_html/store/includes/functions/functions_general.php on line 44

Please help. Thanks.

You're using v1.3.7 without the PayPal patch.

Either upgrade to v1.3.7.1 or v1.3.8a and benefit from a multitude of bugfixes, or install the latest PayPal patch for v1.3.7: http://www.zen-cart.com/forum/showthread.php?t=56216

Great... I've gotten past the error. I have the PayPal Express Checkout set to sandbox for testing. When I try to check out using PayPal to test, I get the following error:

We are sorry for the inconvenience. The PayPal account in this store is presently misconfigured to use mixed sandbox and live settings. We are unable to complete your transaction. Please notify the store owner so they can correct this problem. (10002)

What else do I need to set for it to be fully intesting mode? Thanks.

If you don't have a sandbox account with PayPal, with sandbox users created and API credentials for those sandbox accounts, then ... don't use sandbox mode.

Instead, use live mode for testing, using real accounts. Then just refund the orders when done testing.