diff --git a/scripts/guibuild.py b/scripts/guibuild.py
index e0ac014..1474220 100755
--- a/scripts/guibuild.py
+++ b/scripts/guibuild.py
@@ -446,20 +446,27 @@ def force_macos_user_install(pkg: Path) -> None:
         raise RuntimeError("Unexpected distribution.xml: <domains> not found")
     dist_xml.write_text(new_xml)
 
-    # Edit postinstall inside the (still archived) component pkg.
-    components = [p for p in expanded.glob("*.pkg") if p.is_file()]
+    # Edit postinstall inside the component pkg. Depending on the macOS
+    # version, --expand leaves the component as an archived file (needs a
+    # nested expand/flatten round) or as an already-expanded directory.
+    components = list(expanded.glob("*.pkg"))
     if len(components) != 1:
-        raise RuntimeError(f"Unexpected pkg layout: components={components}")
+        contents = sorted(p.name for p in expanded.iterdir())
+        raise RuntimeError(f"Unexpected pkg layout: components={components} in {contents}")
     component = components[0]
-    comp_dir = expanded / (component.stem + "-component")
-    subprocess.run(["pkgutil", "--expand", str(component), str(comp_dir)], check=True)
+    if component.is_dir():
+        comp_dir = component
+    else:
+        comp_dir = expanded / (component.stem + "-component")
+        subprocess.run(["pkgutil", "--expand", str(component), str(comp_dir)], check=True)
     postinstall = comp_dir / "Scripts" / "postinstall"
     script = postinstall.read_text()
     if 'sudo -u "$USER" ' not in script:
         raise RuntimeError("Unexpected postinstall script: sudo prefix not found")
     postinstall.write_text(script.replace('sudo -u "$USER" ', ""))
-    subprocess.run(["pkgutil", "--flatten", str(comp_dir), str(component)], check=True)
-    shutil.rmtree(comp_dir)
+    if comp_dir is not component:
+        subprocess.run(["pkgutil", "--flatten", str(comp_dir), str(component)], check=True)
+        shutil.rmtree(comp_dir)
 
     subprocess.run(["pkgutil", "--flatten", str(expanded), str(pkg)], check=True)
     shutil.rmtree(expanded)