Webhook Status 500 Method Not Allowed

I've built an endpoint using Google Apps-Script to receive and process incoming webhooks from Bouncie so that I can log the events in Google sheets.  I'm able to receive and log the data, but I can see from the Bouncie dev portal that events being sent to my endpoint are failing with Status 500 "Method Not Allowed", and as a result, Bouncie continues to retry sending the events.  As far as I can tell, my endpoint responds with a dead-simple JSON: 

return ContentService.createTextOutput("{}").setMimeType(ContentService.MimeType.JSON);

Is there a different response format expected by the Bouncie webhook?   What else might be the issue?