diff --git a/apps/counter/api/serializers.py b/apps/counter/api/serializers.py
index b99836d..316a420 100644
--- a/apps/counter/api/serializers.py
+++ b/apps/counter/api/serializers.py
@@ -53,10 +53,16 @@ class DeviceInfoSerializer(serializers.ModelSerializer):
         ]
 
     def get_compile_time(self, obj):
-        return obj.compile_time.strftime('%Y-%m-%d %H:%M:%S')
+        if obj.compile_time:
+            return obj.compile_time.strftime('%Y-%m-%d %H:%M:%S')
+        return None
 
     def get_last_connect(self, obj):
-        return obj.last_connect.strftime('%Y-%m-%d %H:%M:%S')
+        if obj.last_connect:
+            return obj.last_connect.strftime('%Y-%m-%d %H:%M:%S')
+        return None
 
     def get_last_offline_time(self, obj):
-        return obj.last_offline_time.strftime('%Y-%m-%d %H:%M:%S')
+        if obj.last_offline_time:
+            return obj.last_offline_time.strftime('%Y-%m-%d %H:%M:%S')
+        return None