I am trying to generate a client\_token like so: 

```
private static BraintreeGateway gateway = new BraintreeGateway (
            Environment.SANDBOX,
            "merchant_id",
            "public_key",
            "private_key"
    );

@ApiMethod(name = "genClientToken", path = "client_token", httpMethod = ApiMethod.HttpMethod.GET)
public ClientToken genClientToken() {
    return new ClientToken(gateway.clientToken().generate());
}

```

Whenever I do this I get a Braintree Authorization Exception, even though the merchant\_id, public\_key and private\_key are all supplied as put in the Sandbox dashboard.

Why am I getting this `com.braintreegateway.exceptions.AuthenticationException` and how do I fix this?