diff --git a/swh/vault/cookers/directory.py b/swh/vault/cookers/directory.py
index 70d9da0..ebcd346 100644
--- a/swh/vault/cookers/directory.py
+++ b/swh/vault/cookers/directory.py
@@ -1,27 +1,27 @@
 # Copyright (C) 2016  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import tarfile
 import tempfile
 
 from swh.model import hashutil
 from swh.vault.cookers.base import BaseVaultCooker
 from swh.vault.to_disk import DirectoryBuilder
 
 
 class DirectoryCooker(BaseVaultCooker):
     """Cooker to create a directory bundle """
     CACHE_TYPE_KEY = 'directory'
 
     def check_exists(self):
         return not list(self.storage.directory_missing([self.obj_id]))
 
     def prepare_bundle(self):
         with tempfile.TemporaryDirectory(prefix='tmp-vault-directory-') as td:
             directory_builder = DirectoryBuilder(
                 self.storage, td.encode(), self.obj_id)
             directory_builder.build()
-            tar = tarfile.open(fileobj=self.fileobj, mode='w')
-            tar.add(td, arcname=hashutil.hash_to_hex(self.obj_id))
+            with tarfile.open(fileobj=self.fileobj, mode='w:gz') as tar:
+                tar.add(td, arcname=hashutil.hash_to_hex(self.obj_id))
diff --git a/swh/vault/cookers/revision_flat.py b/swh/vault/cookers/revision_flat.py
index 0fc8579..242c998 100644
--- a/swh/vault/cookers/revision_flat.py
+++ b/swh/vault/cookers/revision_flat.py
@@ -1,32 +1,32 @@
 # Copyright (C) 2016-2017  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import tarfile
 import tempfile
 from pathlib import Path
 
 from swh.model import hashutil
 from swh.vault.cookers.base import BaseVaultCooker
 from swh.vault.to_disk import DirectoryBuilder
 
 
 class RevisionFlatCooker(BaseVaultCooker):
     """Cooker to create a revision_flat bundle """
     CACHE_TYPE_KEY = 'revision_flat'
 
     def check_exists(self):
         return not list(self.storage.revision_missing([self.obj_id]))
 
     def prepare_bundle(self):
         with tempfile.TemporaryDirectory(prefix='tmp-vault-revision-') as td:
             root = Path(td)
             for revision in self.storage.revision_log([self.obj_id]):
                 revdir = root / hashutil.hash_to_hex(revision['id'])
                 revdir.mkdir()
                 directory_builder = DirectoryBuilder(
                     self.storage, str(revdir).encode(), revision['directory'])
                 directory_builder.build()
-            tar = tarfile.open(fileobj=self.fileobj, mode='w')
-            tar.add(td, arcname=hashutil.hash_to_hex(self.obj_id))
+            with tarfile.open(fileobj=self.fileobj, mode='w:gz') as tar:
+                tar.add(td, arcname=hashutil.hash_to_hex(self.obj_id))