diff --git a/pristine_zip/reference.py b/pristine_zip/reference.py
index 366757f..058c26c 100644
--- a/pristine_zip/reference.py
+++ b/pristine_zip/reference.py
@@ -1,92 +1,96 @@
 # 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
 
 """Creates a ZIP file deterministically. This can be used a base reference
 for delta files."""
 
 import os
 import subprocess
 
 from . import common
 from . import parameters
 
 
 def normalize_timestamps(entries):
     for entry in entries:
         os.utime(entry, (0, 0))
 
 
 def normalize_perms(checkout_dir):
     # Info-ZIP has an extension that allows storing permissions; but not all
     # implementations support it (eg. not Python). So let's normalize by
     # wiping all permissions.
     for dir_entry in os.scandir(checkout_dir):
         if dir_entry.is_dir(follow_symlinks=False):
             os.chmod(dir_entry.path, 0o100755)
             normalize_perms(dir_entry.path)
         elif dir_entry.is_file(follow_symlinks=False):
             os.chmod(dir_entry.path, 0o100644)
         elif dir_entry.is_symlink():
             os.chmod(dir_entry.path, 0o120000)
         else:
             assert False, f"Unknown DirEntry type: {dir_entry}"
 
 
 def walk(checkout_dir):
     entries = []
     for (dirpath, dirnames, filenames) in os.walk(checkout_dir):
         assert dirpath.startswith(checkout_dir)
         dirpath = dirpath[len(checkout_dir) :].lstrip("/")
         entries.append(dirpath)
         paths = [os.path.join(dirpath, filename) for filename in filenames]
         entries.extend(paths)
     return entries
 
 
 def compress(
     executables: common.Executables,
     encoding_software: parameters.EncodingSoftware,
     checkout_dir: str,
     target: str,
 ):
     """Generates a reference zipball for the given checked out directory."""
     try:
         os.remove(target)
     except FileNotFoundError:
         pass
     assert os.path.isdir(checkout_dir), checkout_dir
 
     entries_str = walk(checkout_dir)
 
     normalize_perms(checkout_dir)
     normalize_timestamps(os.path.join(checkout_dir, entry) for entry in entries_str)
 
     # Encode *before* sorting; sorting on unicode changes across configurations.
     entries = [entry.encode() for entry in entries_str]
 
     # Sort entries ourselves; InfoZIP's zip does not guarantee order
     entries.sort()
 
     if encoding_software == parameters.EncodingSoftware.INFOZIP_3_0:
         # -X = --no-extra, which prevents inclusion of extra non-deterministic
         # and implementation-dependant data
         # -o = --latest-time, which sets the modification time of the zip to that
         # of the most recent file
         assert executables.infozip_3_0
         proc = subprocess.run(
             [executables.infozip_3_0, "-X", "-o", target, "--names-stdin"],
             cwd=checkout_dir,
             input=b"\n".join(entries),
         )
         proc.check_returncode()
     elif encoding_software == parameters.EncodingSoftware.SEVENZIP_6_3:
         assert executables.sevenzip_6_3
         assert target.endswith(".zip")
         proc = subprocess.run(
             [executables.sevenzip_6_3, "a", target, *entries],
             cwd=checkout_dir,
             capture_output=True,
         )
         proc.check_returncode()
+
+        # 7zip doesn't provide a way to disable its nondeterministic fields;
+        # let's remove them after the fact.
+        proc = subprocess.run(["strip-nondeterminism", "-t", "zip", target])