diff --git a/python/paddle/fluid/layers/nn.py b/python/paddle/fluid/layers/nn.py
index 4d8f887cbaa751eb8eda83c8ec3e45cf137366b8..f4f462ec3bbae03a8f5e87e3083a0ee26a51a72b 100644
--- a/python/paddle/fluid/layers/nn.py
+++ b/python/paddle/fluid/layers/nn.py
@@ -6689,6 +6689,7 @@ def clip_by_norm(x, max_norm, name=None):
         out = helper.create_variable(
             name=name, dtype=x.dtype, persistable=False)
 
+    # max_norm should always be set
     helper.append_op(
         type="clip_by_norm",
         inputs={"X": x},