diff --git a/api/operation.py b/api/operation.py
index a10e413..48482fa 100644
--- a/api/operation.py
+++ b/api/operation.py
@@ -14,7 +14,7 @@ def _remove_operation(name: str, id: int) -> None:
     if row != None:
         raise Exception('Disallow to remove parent operation !')
 
-    sql += f'delete from snapshot_operation where id = {id};'
+    sql = f'delete from snapshot_operation where id = {id};'
     sql += f'delete from operation where id = {id}'
     write(name, sql)