diff --git a/examples/client/main.cpp b/examples/client/main.cpp
index 0f4dfc7..fe43de1 100644
--- a/examples/client/main.cpp
+++ b/examples/client/main.cpp
@@ -344,7 +344,7 @@ void test_callback() {
 		}, test);
 
 		std::string test1 = "test" + std::to_string(i + 2);
-		//zero means no timeout check
+		//zero means no timeout check, no param means using default timeout(5s)
 		client.async_call<0>("echo", [](const boost::system::error_code & ec, string_view data) {
 			req_result result(data);
 			auto str = result.as<std::string>();