I have successfully set up the Stripe gateway, however when I try to submit the payment as a user, I get the error, "The boolean sandbox option must be set." I do not get this error with PayPal, only with Stripe.

Here are my relevant config.yml entries:

payum:
    gateways:
        paypal_express_checkout:
            factory: "paypal_express_checkout"
            payum.http_client: "@sylius.payum.http_client"
            username: "%paypal.express_checkout.username%"
            password: "%paypal.express_checkout.password%"
            signature: "%paypal.express_checkout.signature%"
            sandbox: true
        stripe:
            factory: stripe_checkout
            publishable_key: pk_test_1UyHCW3tydYjdxYLvaGPRMKh
            secret_key: sk_test_81j9puWh2fTnROgOtIK0hjLb
            sandbox: true

sylius_payment:
     driver: doctrine/orm
     gateways:
        stripe: Stripe

What am I doing wrong?

(Also, those are not the real keys)