@@ -71,13 +71,21 @@
 
 - (void)of_addObject: (id)object
       fileDescriptor: (int)fd
 	      events: (short)events
 {
-	struct pollfd *FDs = _FDs.mutableItems;
-	size_t count = _FDs.count;
-	bool found = false;
+	struct pollfd *FDs;
+	size_t count;
+	bool found;
+
+	if (fd < 0)
+		@throw [OFObserveFailedException exceptionWithObserver: self
+								 errNo: EBADF];
+
+	FDs = _FDs.mutableItems;
+	count = _FDs.count;
+	found = false;
 
 	for (size_t i = 0; i < count; i++) {
 		if (FDs[i].fd == fd) {
 			FDs[i].events |= events;
 			found = true;
@@ -102,12 +110,19 @@
 
 - (void)of_removeObject: (id)object
 	 fileDescriptor: (int)fd
 		 events: (short)events
 {
-	struct pollfd *FDs = _FDs.mutableItems;
-	size_t nFDs = _FDs.count;
+	struct pollfd *FDs;
+	size_t nFDs;
+
+	if (fd < 0)
+		@throw [OFObserveFailedException exceptionWithObserver: self
+								 errNo: EBADF];
+
+	FDs = _FDs.mutableItems;
+	nFDs = _FDs.count;
 
 	for (size_t i = 0; i < nFDs; i++) {
 		if (FDs[i].fd == fd) {
 			FDs[i].events &= ~events;