diff --git a/src/event/SocketEvent.cxx b/src/event/SocketEvent.cxx
index 98313a6b0..ad2aa576a 100644
--- a/src/event/SocketEvent.cxx
+++ b/src/event/SocketEvent.cxx
@@ -31,8 +31,9 @@
 void
 SocketEvent::Open(SocketDescriptor _fd) noexcept
 {
-	assert(!fd.IsDefined());
 	assert(_fd.IsDefined());
+	assert(!fd.IsDefined());
+	assert(GetScheduledFlags() == 0);
 
 	fd = _fd;
 }