Flatpak Builder does ignore --build-shell parameter?

[Lothar Scholz]

Why is the following not working?

flatpak-builder --build-shell=program build_example
example/flatpak_manifest.yaml

It copies the sources but does not stop and instead is building it
until an error happened.
But it works with stop-at (so there is no spelling error)

flatpak-builder --stop-at=program build_example example/flatpak_manifest.yaml