diff --git a/src/WebSocketContextData.h b/src/WebSocketContextData.h
index ccd1530..9301f3d 100644
--- a/src/WebSocketContextData.h
+++ b/src/WebSocketContextData.h
@@ -32,6 +32,8 @@
 namespace uWS {
 
 template <bool, bool, typename> struct WebSocket;
+struct TopicTreeMessage;
+struct TopicTreeBigMessage;
 
 /* todo: this looks identical to WebSocketBehavior, why not just std::move that entire thing in? */