Configure Bind9 to forward for a zone and reject all other requests

I have an interesting use case where my hosting providers (Heroku) DNS has intermittent issues resolving a certain subdomain.

They have an option that allows you to insert a custom resolver as the primary lookup mechanism and then any failed requests will fallback to their DNS.

How might I setup a bind9 server to forward requests to 8.8.8.8 for one subdomain only? And all other requests need to fail, causing the client to use the secondary nameserver (which I don't know the IP of).

My first attempt was to add the zone and specify the forwarder;

zone "a-test.subdomain.com" {
    type forward;
    forwarders {
        8.8.8.8;
    };
};

And then setting

recursion no;
allow-query { any; };

But it results in

query failed (REFUSED) for a-test.subdomain.com/IN/A at query.c:5665

Inside the configuration file BIND9, locate the options section and add or modify the following lines:

options {
    recursion yes;
    allow-query { any; };
};

Below the options section, add the following lines:

view "forward-view" {
    match-clients { any; };
    recursion no;

    zone "a-test.subdomain.com" {
        type forward;
        forward only;
        forwarders {
            8.8.8.8;
        };
    };
};

view "fallback-view" {
    match-clients { any; };
    recursion no;

    // Add your secondary nameserver's IP address here
    // Replace 192.0.2.2 with the actual IP address
    zone "." IN {
        type forward;
        forwarders {
            192.0.2.2;
        };
    };
};

Save the changes and restart the BIND9 service to apply the new configuration.