diff --git a/packages/server/src/store.ts b/packages/server/src/store.ts
index 879241a..2d95864 100644
--- a/packages/server/src/store.ts
+++ b/packages/server/src/store.ts
@@ -150,7 +150,10 @@ export function ledgerStat(roomId: string): { bytes: number; mtimeMs: number } |
 // the room's whole truth and the room stays asleep, unreplayed.
 
 const stubDir = () => join(DATA_DIR, "..", "stubs");
-const stubFor = (roomId: string) => join(stubDir(), `${roomId}.json`);
+function stubFor(roomId: string): string {
+  if (!safeRoomId(roomId)) throw new Error(`unsafe room id: ${JSON.stringify(roomId)}`);
+  return join(stubDir(), `${roomId}.json`);
+}
 
 export function readStubFile<T>(roomId: string): { bytes: number; stub: T } | null {
   if (!safeRoomId(roomId)) return null;