loopback async/await UnhandledPromiseRejectionWarning issue

In loopback 3 When i implement async/await in the before save Operation hook

Entry.observe('before save', async (ctx, next) =>{

    if(ctx.instance.images){
        Entry.upload(ctx.instance.images[0].src).then(data => {
            ctx.instance.image = data;
        });
    }
});

the console print this error

(node:29323) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: Callback was already called.
(node:29323) DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

How can i get rid of this issue!?

PS. i cant figure out where's that promise, and this message disappear once i remove the async/await.

via dyaa