When the webhook is fired I got this err: "topic.toUpperCase is not a function!

[kacherHuynh]

Hi all, 

I have tried to set up a webhook to fire when the users publish theme. 

But when the webhook is fired, I got an error as below: 

"topic.toUpperCase is not a function at receiveWebhookMiddleware (/root/my-app/node_modules/@shopify/koa-shopify-webhooks/build/node/receive.js:37:32)"

Here are modules I use for my app: 

"koa": "^2.13.0",
"koa-bodyparser": "^4.3.0",
"koa-http-request": "^2.2.1",
"koa-router": "^8.0.8",
"koa-session": "^6.1.0"

As I research around, some people say that the problem could be the bodyParser, but even I remove the bodyParser by commenting out the code for bodyParser part, the err is still happened. 

Please help 😞