diff --git a/tests/resources/encrypted_algo4.pdf b/tests/resources/encrypted_algo4.pdf
new file mode 100644
index 00000000..bc21948a
Binary files /dev/null and b/tests/resources/encrypted_algo4.pdf differ
diff --git a/tests/test_main.py b/tests/test_main.py
index 47f3cdd8..266a1a36 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -547,3 +547,7 @@ def test_old_unpaper():
         'c02-22.pdf', 'wont_be_created.pdf', '--clean', env=env)
     assert sh.returncode == ExitCode.missing_dependency
 
+
+def test_algo4():
+    sh, _, _ = run_ocrmypdf_env('encrypted_algo4.pdf', 'wontwork.pdf')
+    assert sh.returncode == ExitCode.encrypted_pdf