diff --git a/pristine_zip/delta_to_zipball.py b/pristine_zip/delta_to_zipball.py
index 96c7d7f..10ad728 100644
--- a/pristine_zip/delta_to_zipball.py
+++ b/pristine_zip/delta_to_zipball.py
@@ -1,61 +1,62 @@
 # 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 compress
 
 
 def genzip(checkout_dir: str, delta_path: str, zipball_path: str):
     with tempfile.TemporaryDirectory(prefix="pristine-zip-genzip") as work_dir:
         reference_zipball_path = os.path.join(work_dir, "reference.zip")
 
         # generate reference zipball
         compress.compress(checkout_dir, reference_zipball_path)
 
         _apply_delta(reference_zipball_path, zipball_path, work_dir, delta_path)
 
 
 def _apply_delta(
     reference_zipball_path: str, zipball_path: str, work_dir: str, delta_path: str,
 ):
     proc = subprocess.run(["tar", "--extract", "-f", delta_path,], cwd=work_dir)
     proc.check_returncode()
 
     with open(os.path.join(work_dir, "type"), "rb") as fd:
         type_ = fd.read().decode().strip()
         assert type_ == "zip", (
             f"Unknown zipball type {type}. Are you"
             f"extracting a delta from pristine-tar instead of pristine-zip?"
         )
 
     with open(os.path.join(work_dir, "reference_md5sum"), "rb") as fd:
         expected_md5sum = fd.read().decode().strip()
 
     proc = subprocess.run(["md5sum", reference_zipball_path], capture_output=True)
     proc.check_returncode()
-    actual_md5sum = proc.stdout.decode().strip()
+    actual_md5sum = proc.stdout.decode().split(" ", 1)[0].strip()
 
     if actual_md5sum != expected_md5sum:
         print(
-            f"md5sum mismatch between reference zipballs. "
+            f"md5sum mismatch between reference zipballs "
+            f"(expected '{expected_md5sum}', got '{actual_md5sum}').\n"
             f"This is a bug, please report it along with the original zipball "
             f"and the version number of pristine-zip."
         )
         exit(1)
 
     xdelta3_path = os.path.join(work_dir, "delta")
     assert os.path.isfile(xdelta3_path), "Missing 'delta' file in delta archive."
 
     _apply_xdelta3(reference_zipball_path, zipball_path, xdelta3_path)
 
 
 def _apply_xdelta3(reference_zipball_path: str, zipball_path: str, xdelta3_path: str):
     proc = subprocess.run(
         ["xdelta3", "-d", "-s", reference_zipball_path, xdelta3_path, zipball_path]
     )
     proc.check_returncode()