diff --git a/http_examples.cpp b/http_examples.cpp
index 2928707..833f25e 100644
--- a/http_examples.cpp
+++ b/http_examples.cpp
@@ -210,7 +210,7 @@ int main() {
     }
   };
 
-  server->on_error = [](shared_ptr<HttpServer::Request> /*request*/, const SimpleWeb::error_code /*ec*/) {
+  server->on_error = [](shared_ptr<HttpServer::Request> /*request*/, const SimpleWeb::error_code & /*ec*/) {
     // handle errors here
   };
 
diff --git a/server_http.hpp b/server_http.hpp
index 6d51186..c17a178 100644
--- a/server_http.hpp
+++ b/server_http.hpp
@@ -337,7 +337,7 @@ namespace SimpleWeb {
 
     std::map<std::string, std::function<void(std::shared_ptr<typename ServerBase<socket_type>::Response>, std::shared_ptr<typename ServerBase<socket_type>::Request>)>> default_resource;
 
-    std::function<void(std::shared_ptr<typename ServerBase<socket_type>::Request>, const error_code)> on_error;
+    std::function<void(std::shared_ptr<typename ServerBase<socket_type>::Request>, const error_code &)> on_error;
 
     std::function<void(std::shared_ptr<socket_type>, std::shared_ptr<typename ServerBase<socket_type>::Request>)> on_upgrade;