diff --git a/Include/genobject.h b/Include/genobject.h
index 1ecd7ad..ca84432 100644
--- a/Include/genobject.h
+++ b/Include/genobject.h
@@ -13,6 +13,7 @@
 	PyObject_HEAD
 	/* The gi_ prefix is intended to remind of generator-iterator. */
 
+	/* Note: gi_frame can be NULL if the generator is "finished" */
 	struct _frame *gi_frame;
 
 	/* True if generator is being executed. */