Forcing HTTPS in Symfony PHP causes redirect loop behind AWS load balancer

I am trying to force HTTPS in my Symfony PHP application. The docs on this topic seem pretty straight-forward:

# config/routes.yamlsecure:    path:       /secure    controller: App\Controller\MainController::secure    schemes:    [https]

Basically just add schemes: [https] to the route.

But when I do this in my application and deploy (to an AWS EC2 instance using a load balancer) it appears to create a redirect loop.

What might be the issue here?