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## Checkout no longer returns an error for a malformed cart ID

Requests to the checkout page that send `cart_id` as an array, for example `cart_id[]=abc`, no longer return an HTTP 500 error.

* **Fixed:** the checkout page now ignores a `cart_id` value that is not a single string and continues with the shopper's primary cart.
* **Unchanged:** requests that send a valid string `cart_id` behave as before, including the redirect to the cart when the ID is not in the shopper's session.