diff --git a/src/models/event-timeline-set.js b/src/models/event-timeline-set.js
index 7ef2ef37682cdb09e42a0b6b32661b96e4e199b9..37e77e17bd8cb79186f12d5714c0cd2af06daec8 100644
--- a/src/models/event-timeline-set.js
+++ b/src/models/event-timeline-set.js
@@ -68,7 +68,7 @@ function EventTimelineSet(room, opts) {
     // just a list - *not* ordered.
     this._timelines = [this._liveTimeline];
     this._eventIdToTimeline = {};
-    this._threadId = opts.threadId;
+    this.threadId = opts.threadId;
     this._filter = opts.filter || null;
 }
 utils.inherits(EventTimelineSet, EventEmitter);
diff --git a/src/models/event-timeline.js b/src/models/event-timeline.js
index cad8cb7e11e42e422235e8fdb2443f520016f1f1..de6f5885ade98f2496bafe427e32777b6c21bf99 100644
--- a/src/models/event-timeline.js
+++ b/src/models/event-timeline.js
@@ -302,7 +302,7 @@ EventTimeline.prototype.addEvent = function(event, atStart) {
 
     // only call setEventMetadata on the unfiltered timelineSets
     const timelineSet = this.getTimelineSet();
-    if (timelineSet._threadId ||  (timelineSet.room &&
+    if (timelineSet.threadId ||  (timelineSet.room &&
         timelineSet.room.getUnfilteredTimelineSet() === timelineSet)) {
         EventTimeline.setEventMetadata(event, stateContext, atStart);