diff --git a/ustream.h b/ustream.h
index d9f98b7..634a3e7 100644
--- a/ustream.h
+++ b/ustream.h
@@ -52,7 +52,7 @@ struct ustream {
 	struct ustream *next;
 
 	/*
-	 * notify_read:
+	 * notify_read: (optional)
 	 * called by the ustream core to notify that new data is available
 	 * for reading.
 	 * must not free the ustream from this callback