diff --git a/src/net/network-monitor.hpp b/src/net/network-monitor.hpp
index afafd58..9b2d08d 100644
--- a/src/net/network-monitor.hpp
+++ b/src/net/network-monitor.hpp
@@ -133,6 +133,8 @@
 class NetworkMonitorImpl : noncopyable
 {
 public:
+  using Error = NetworkMonitor::Error;
+
   virtual
   ~NetworkMonitorImpl() = default;