diff --git a/app.py b/app.py
index 3e561ae..81dae85 100644
--- a/app.py
+++ b/app.py
@@ -13,7 +13,8 @@ def sigterm_handler_wrap(_signo):
     def func(_signo, _stack_frame):
         logger.info("signal {} received, exiting...".format(_signo))
         conf().save_user_datas()
-        return old_handler(_signo, _stack_frame)
+        if callable(old_handler): #  check old_handler
+            return old_handler(_signo, _stack_frame)
     signal.signal(_signo, func)
 
 def run():