diff --git a/python/paddle/v2/fluid/backward.py b/python/paddle/v2/fluid/backward.py
index ae81d68bafd22db5d9f7ab0f9cc0dcdb204493e1..29243c90e872ca4a7d1ce6f84f6297b865655da1 100644
--- a/python/paddle/v2/fluid/backward.py
+++ b/python/paddle/v2/fluid/backward.py
@@ -178,7 +178,7 @@ def _remove_no_grad_branch_(op_descs, no_grad_set):
         if _all_in_set_(
                 filter(lambda name: name.find(core.grad_var_suffix()) != -1,
                        op_desc.input_arg_names()), no_grad_set):
-            no_grad_set.union(out_arg_names)
+            no_grad_set.update(out_arg_names)
             return True
         return False