diff --git a/python/examples/bert/bert_web_service.py b/python/examples/bert/bert_web_service.py
index 8db64e5eb792a7365ed739bbfb05bf38fd8a0da1..6a5830ea179b033f9f761010d8cf9213d9b1e40b 100644
--- a/python/examples/bert/bert_web_service.py
+++ b/python/examples/bert/bert_web_service.py
@@ -23,10 +23,10 @@ class BertService(WebService):
     def load(self):
         self.reader = BertReader(vocab_file="vocab.txt", max_seq_len=128)
 
-    def preprocess(self, feed={}, fetch=[]):
-        feed_res = [{
-            "words": self.reader.process(ins["words"].encode("utf-8"))
-        } for ins in feed]
+    def preprocess(self, feed=[], fetch=[]):
+        feed_res = [
+            self.reader.process(ins["words"].encode("utf-8")) for ins in feed
+        ]
         return feed_res, fetch