diff --git a/sync_watch.cpp b/sync_watch.cpp
index bb2a860..c0f0f21 100644
--- a/sync_watch.cpp
+++ b/sync_watch.cpp
@@ -73,7 +73,7 @@
     }
 }
 
-int SyncWatch::callback(sd_event_source* s, int fd, uint32_t revents,
+int SyncWatch::callback(sd_event_source* /* s */, int fd, uint32_t revents,
                         void* userdata)
 {
     if (!(revents & EPOLLIN))