diff --git a/roomserver/api/wrapper.go b/roomserver/api/wrapper.go
index 2ebe2f648..f56f66556 100644
--- a/roomserver/api/wrapper.go
+++ b/roomserver/api/wrapper.go
@@ -54,6 +54,13 @@ func SendEventWithState(
 		return err
 	}
 
+	// The roomserver processes events strictly in the order they are given,
+	// so we need to make sure that events don't refer to auth events that
+	// the roomserver doesn't know about yet.
+	outliers = gomatrixserverlib.ReverseTopologicalOrdering(
+		outliers, gomatrixserverlib.TopologicalOrderByAuthEvents,
+	)
+
 	var ires []InputRoomEvent
 	for _, outlier := range outliers {
 		if haveEventIDs[outlier.EventID()] {