Wait for asynchronous request on background page

Question

sendResponse is not waiting for fetch to complete.

chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
    async function f() {
        const e  = await fetch('a.txt')
        return await e.text()
    }
    
    f().then(r => {
        sendResponse({res: r})
    })
})

On the content.js page the following error is shown:

Uncaught (in promise) Error: The message port closed before a response was received.

Where is the error?