diff --git a/_update-charmcraft.py b/_update-charmcraft.py
index 3354e62..86238c3 100755
--- a/_update-charmcraft.py
+++ b/_update-charmcraft.py
@@ -87,7 +87,10 @@ def cc3ify(args: argparse.Namespace, charmcraft: Any) -> Any:
     charmcraft['platforms'] = {}
     platforms = args.platforms.split(',')
     for platform in platforms:
-        charmcraft['platforms'][platform] = ""
+        charmcraft['platforms'][platform] = {
+            "build-on": platform,
+            "build-for": platform,
+        }
 
     return charmcraft