--- a/services/sync/modules/engines.js
+++ b/services/sync/modules/engines.js
@@ -832,17 +832,17 @@ SyncEngine.prototype = {
 
   // Which sortindex to use when retrieving records for this engine.
   _defaultSort: undefined,
 
   _metadataPostProcessor(json) {
     if (Array.isArray(json)) {
       // Pre-`JSONFile` storage stored an array, but `JSONFile` defaults to
       // an object, so we wrap the array for consistency.
-      return { ids: json };
+      json = { ids: json };
     }
     if (!json.ids) {
       json.ids = [];
     }
     // The set serializes the same way as an array, but offers more efficient
     // methods of manipulation.
     json.ids = new SerializableSet(json.ids);
     return json;