diff --git a/lib/utils/migration-utils.js b/lib/utils/migration-utils.js
--- a/lib/utils/migration-utils.js
+++ b/lib/utils/migration-utils.js
@@ -1,5 +1,7 @@
 // @flow
 
+import invariant from 'invariant';
+
 import type { TranslatedThreadMessageInfos } from './message-ops-utils.js';
 import { entries } from './objects.js';
 import { ashoatKeyserverID } from './validation-utils.js';
@@ -23,7 +25,7 @@
   threadPermissionPropagationPrefixes,
   threadPermissionFilterPrefixes,
 } from '../types/thread-permission-types.js';
-import type { ThreadStoreThreadInfos } from '../types/thread-types.js';
+import type { RawThreadInfos } from '../types/thread-types.js';
 
 function convertDraftKeyToNewIDSchema(key: string): string {
   const threadID = key.slice(0, -draftKeySuffix.length);
@@ -67,13 +69,19 @@
 }
 
 function convertThreadStoreThreadInfosToNewIDSchema(
-  threadStoreThreadInfos: ThreadStoreThreadInfos,
-): ThreadStoreThreadInfos {
+  threadStoreThreadInfos: RawThreadInfos,
+): RawThreadInfos {
   return Object.fromEntries(
-    entries(threadStoreThreadInfos).map(([id, threadInfo]) => [
-      `${ashoatKeyserverID}|` + id,
-      convertRawThreadInfoToNewIDSchema(threadInfo),
-    ]),
+    entries(threadStoreThreadInfos).map(([id, threadInfo]) => {
+      invariant(
+        !threadInfo.minimallyEncoded,
+        'threadInfo must not be minimallyEncoded',
+      );
+      return [
+        `${ashoatKeyserverID}|` + id,
+        convertRawThreadInfoToNewIDSchema(threadInfo),
+      ];
+    }),
   );
 }