Addressed PR comments.
diff --git a/python/repeated.h b/python/repeated.h
index 9e8530b..8204afd 100644
--- a/python/repeated.h
+++ b/python/repeated.h
@@ -34,6 +34,7 @@
 #include "upb/def.h"
 
 // Creates a new repeated field stub for field `f` of message object `parent`.
+// Precondition: `parent` must be a stub.
 PyObject* PyUpb_RepeatedContainer_NewStub(PyObject* parent,
                                           const upb_fielddef* f,
                                           PyObject* arena);