diff --git a/app/eva_common.py b/app/eva_common.py
index f05f45b..9d658c7 100644
--- a/app/eva_common.py
+++ b/app/eva_common.py
@@ -638,6 +638,8 @@ def gen_flow_turn_rate_index(avg_cross_delay_info, roads_dir_dict):
 
 
 def calc_service_level(delay_time):
+    if delay_time == '-' or delay_time == '':
+        return '-'
     if delay_time <= 10:
         service_level = 'A'
     elif 10 < delay_time <= 20: