diff --git a/src/sandstorm/grain.capnp b/src/sandstorm/grain.capnp
index 7c5f9174f0..becb00834c 100644
--- a/src/sandstorm/grain.capnp
+++ b/src/sandstorm/grain.capnp
@@ -613,7 +613,7 @@ struct GrainInfo {
 # ========================================================================================
 # Persistent objects
 
-interface AppPersistent(AppObjectId) {
+interface AppPersistent @0xaffa789add8747b8 (AppObjectId) {
   # To make an object implemented by your own app persistent, implement this interface.
   #
   # `AppObjectId` is a structure like a URL which identifies a specific object within your app.