diff --git a/fastchat/serve/monitor/basic_stats.py b/fastchat/serve/monitor/basic_stats.py
index 3c1a8793d..c8057f14c 100644
--- a/fastchat/serve/monitor/basic_stats.py
+++ b/fastchat/serve/monitor/basic_stats.py
@@ -36,6 +36,7 @@ def get_log_files(max_num_files=None):
 
 def load_log_files(filename):
     data = []
+    lines = []
     for retry in range(5):
         try:
             lines = open(filename).readlines()