diff --git a/http/routing/ruleparametertraits.hpp b/http/routing/ruleparametertraits.hpp
index 949f390..3ed9aa0 100644
--- a/http/routing/ruleparametertraits.hpp
+++ b/http/routing/ruleparametertraits.hpp
@@ -13,6 +13,11 @@
 template <typename T>
 struct RuleParameterTraits
 {
+  private:
+    RuleParameterTraits() = default;
+    friend T;
+
+  public:
     using self_t = T;
     WebSocketRule& websocket()
     {