diff --git a/sys/kern/waitchannel.c b/sys/kern/waitchannel.c
index 941751b..1d50547 100644
--- a/sys/kern/waitchannel.c
+++ b/sys/kern/waitchannel.c
@@ -92,9 +92,11 @@ WaitChannel_Wake(WaitChannel *wchan)
     Spinlock_Lock(&wchan->lock);
 
     thr = TAILQ_FIRST(&wchan->chanQueue);
-    TAILQ_REMOVE(&wchan->chanQueue, thr, chanQueue);
-    Sched_SetRunnable(thr);
-    Thread_Release(thr);
+    if (thr != NULL) {
+	TAILQ_REMOVE(&wchan->chanQueue, thr, chanQueue);
+	Sched_SetRunnable(thr);
+	Thread_Release(thr);
+    }
 
     Spinlock_Unlock(&wchan->lock);
 }