diff --git a/src/components/structures/TimelinePanel.js b/src/components/structures/TimelinePanel.js
index 4fbca4d40a..18f52d1f07 100644
--- a/src/components/structures/TimelinePanel.js
+++ b/src/components/structures/TimelinePanel.js
@@ -734,7 +734,7 @@ var TimelinePanel = React.createClass({
 
         // the messagePanel doesn't know where the read marker is.
         // if we know the timestamp of the read marker, make a guess based on that.
-        var rmTs = TimelinePanel.roomReadMarkerTsMap[this.props.timelineSet.roomId];
+        const rmTs = TimelinePanel.roomReadMarkerTsMap[this.props.timelineSet.room.roomId];
         if (rmTs && this.state.events.length > 0) {
             if (rmTs < this.state.events[0].getTs()) {
                 return -1;