Electron-forge flatpak maker exits with status code 1

When I try to run electron-forge make with @electron-forge/maker-flatpak, it fails with an error:

An unhandled error has occurred inside Forge:
An error occured while making for target: flatpak
flatpak failed with status code 1
Error: flatpak failed with status code 1
    at ChildProcess.<anonymous> (/home/trusktr/src/foo+bar/node_modules/@malept/flatpak-bundler/index.js:71:16)
    at ChildProcess.emit (events.js:315:20)
    at ChildProcess.EventEmitter.emit (domain.js:485:12)
    at maybeClose (internal/child_process.js:1051:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:287:5)

This is my electron-forge config:

module.exports = {
  packagerConfig: {},
  makers: [ {name: '@electron-forge/maker-flatpak'} ],
}

Any ideas?

Moved to GitHub: https://github.com/electron-userland/electron-forge/issues/1792