diff --git a/pristine_zip/zipball_to_delta.py b/pristine_zip/zipball_to_delta.py
index ba0d34a..6d8463b 100644
--- a/pristine_zip/zipball_to_delta.py
+++ b/pristine_zip/zipball_to_delta.py
@@ -1,97 +1,97 @@
 # Copyright (C) 2020 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 os
 import subprocess
 import tempfile
 
 from . import reference
 
 
 def gendelta(zipball_path: str, delta_path: str):
     with tempfile.TemporaryDirectory(prefix="pristine-zip-gendelta") as work_dir:
         reference_zipball_path = os.path.join(work_dir, "reference.zip")
         generate_reference_zipball_from_zipball(
             zipball_path, reference_zipball_path, work_dir
         )
         _generate_delta(zipball_path, reference_zipball_path, work_dir, delta_path)
 
 
 def _generate_delta(
     upstream_zipball_path: str,
     reference_zipball_path: str,
     work_dir: str,
     delta_path: str,
 ):
     _generate_xdelta3(
         upstream_zipball_path, reference_zipball_path, os.path.join(work_dir, "delta")
     )
     with open(os.path.join(work_dir, "type"), "wb") as fd:
         fd.write(b"zip\n")
 
     proc = subprocess.run(["md5sum", reference_zipball_path], capture_output=True)
     proc.check_returncode()
     md5sum = proc.stdout.split(b" ", 1)[0]
 
     with open(os.path.join(work_dir, "reference_md5sum"), "wb") as fd:
         fd.write(md5sum + b"\n")
 
     files = ["delta", "reference_md5sum", "type"]
 
     # Make the timestamps in the delta tarball deterministic
     for file in files:
         os.utime(os.path.join(work_dir, file), times=(0, 0))
 
     proc = subprocess.run(
         [
             "tar",
             # make entries in the delta tarball deterministic:
             "--owner",
             "0",
             "--group",
             "0",
             "--numeric-owner",
             "--mode",
             "644",
             # generic options:
             "--create",
-            "--compress",
+            "--gzip",
             "-f",
             delta_path,
             *files,
         ],
         cwd=work_dir,
     )
     proc.check_returncode()
 
 
 def _generate_xdelta3(
     upstream_zipball_path: str, reference_zipball_path: str, delta_path: str
 ):
     """Generates the xdelta3 difference between a reference zipball and
     the original one."""
     proc = subprocess.run(
         [
             "xdelta3",
             "-e",
             "-s",
             reference_zipball_path,
             upstream_zipball_path,
             delta_path,
         ]
     )
     proc.check_returncode()
 
 
 def generate_reference_zipball_from_zipball(
     upstream_zipball_path: str, reference_zipball_path: str, work_dir: str
 ):
     """Unzips an upstream zipball and rezips it in a reference zipball."""
     checkout_dir = os.path.join(work_dir, "checkout")
     os.mkdir(checkout_dir)
     proc = subprocess.run(["unzip", upstream_zipball_path], cwd=checkout_dir)
     proc.check_returncode()
 
     reference.compress(checkout_dir, reference_zipball_path)