Reverb issue in PusherBroadcaster

Hi all. I'm working to get reverb up and running in our system and have hit a strange error. It looks like things are set up correctly - when I start the reverb server (using the --debug option) I see that it's subscribed to all the channels that I'm working with, and I see the pusher pings coming through. When I dispatch an event on the channel reverb is listening to, I get the following 405 error:

ERROR Uncaught exception: Pusher error: Method not allowed.. in /vendor/laravel/framework/src/Illuminate/Broadcasting/Broadcasters/PusherBroadcaster.php:164

It seems like everything is working, but reverb is attempting to send a $_GET transmission to a $_POST endpoint and I don't see how I have any control over it. Anyone have any ideas on what's actually happening? We're using Apache on AWS if that makes any difference.