diff --git a/examples/asio-cl.cc b/examples/asio-cl.cc
index b8908265..a96de1c2 100644
--- a/examples/asio-cl.cc
+++ b/examples/asio-cl.cc
@@ -103,8 +103,8 @@ int main(int argc, char *argv[]) {
       });
     });
 
-    sess.on_error([](const std::string &error) {
-      std::cerr << "error: " << error << std::endl;
+    sess.on_error([](const boost::system::error_code &ec) {
+      std::cerr << "error: " << ec.message() << std::endl;
     });
 
     io_service.run();
diff --git a/src/asio_client_session_impl.cc b/src/asio_client_session_impl.cc
index 58985380..cf7c049d 100644
--- a/src/asio_client_session_impl.cc
+++ b/src/asio_client_session_impl.cc
@@ -91,7 +91,7 @@ void session_impl::connected() {
 void session_impl::not_connected(const boost::system::error_code &ec) {
   auto &error_cb = on_error();
   if (error_cb) {
-    error_cb(ec.message());
+    error_cb(ec);
   }
 }
 
@@ -307,7 +307,7 @@ bool session_impl::setup_session() {
   if (rv != 0) {
     auto &error_cb = on_error();
     if (error_cb) {
-      error_cb(nghttp2_strerror(rv));
+      error_cb(make_error_code(static_cast<nghttp2_error>(rv)));
     }
     return false;
   }
diff --git a/src/includes/nghttp2/asio_http2.h b/src/includes/nghttp2/asio_http2.h
index 8c2a8033..6a2d7d82 100644
--- a/src/includes/nghttp2/asio_http2.h
+++ b/src/includes/nghttp2/asio_http2.h
@@ -76,7 +76,7 @@ const boost::system::error_category &nghttp2_category() noexcept;
 
 typedef std::function<void(const uint8_t *, std::size_t)> data_cb;
 typedef std::function<void(void)> void_cb;
-typedef std::function<void(const std::string &err)> error_cb;
+typedef std::function<void(const boost::system::error_code &ec)> error_cb;
 typedef std::function<void(uint32_t)> close_cb;
 
 // Callback function to generate response body.  The implementation of