diff --git a/ustream.c b/ustream.c
index ed4fbb3..8e6efcd 100644
--- a/ustream.c
+++ b/ustream.c
@@ -104,7 +104,8 @@ static void ustream_state_change_cb(struct uloop_timeout *t)
 
 	if (s->write_error)
 		ustream_free_buffers(&s->w);
-	s->notify_state(s);
+	if (s->notify_state)
+		s->notify_state(s);
 }
 
 void ustream_init_defaults(struct ustream *s)
diff --git a/ustream.h b/ustream.h
index 634a3e7..2df429c 100644
--- a/ustream.h
+++ b/ustream.h
@@ -68,7 +68,7 @@ struct ustream {
 	void (*notify_write)(struct ustream *s, int bytes);
 
 	/*
-	 * notify_state:
+	 * notify_state: (optional)
 	 * called by the ustream implementation to notify that the read
 	 * side of the stream is closed (eof is set) or there was a write
 	 * error (write_error is set).