This may only be related to the macOS-specific part of the code.

I am using the example code from the README:

ipcMain.on('download-request', async (event, {url}) => {
    const win = BrowserWindow.getFocusedWindow();
    console.log(await download(win, url));
});

If I run ipcRenderer.send({url: 'https://google.com'}) the download works well.
If I run something bogus like ipcRenderer.send({url: 'a'}), I get the above error; moreover, if I now run ipcRenderer.send({url: 'https://google.com'}), I keep getting this error (even though the file is still downloaded in the background, it seems, without bouncing the downloads folder icon). Only restarting the main process stops these errors.