diff --git a/python/paddle/fluid/layers/io.py b/python/paddle/fluid/layers/io.py
index e8eff33bd172ce5c4fe2ad336b81a18ca1005252..acfad45704d4ea9e28711c019db3563489aab3ff 100644
--- a/python/paddle/fluid/layers/io.py
+++ b/python/paddle/fluid/layers/io.py
@@ -170,7 +170,7 @@ class ListenAndServ(object):
                 'OptimizeBlock': current_block,
                 'PrefetchBlock': empty_block,
                 'sync_mode': True,  # did not support async now in layers
-                'grad_to_block_id': []
+                'grad_to_block_id': [""]
             })