diff --git a/app/models/comment.js b/app/models/comment.js
index d9d93e5..ec0a0e1 100644
--- a/app/models/comment.js
+++ b/app/models/comment.js
@@ -44,7 +44,7 @@ CommentSchema.index({
   resource: 1,
   replyTo: 1,
 }, {
-  partialFilterExpression: { $exists: { replyTo: 1 } },
+  partialFilterExpression: { replyTo: { $exists: true } },
   name: 'comment_replies',
 });