diff --git a/hate_crack/main.py b/hate_crack/main.py
index 19cba8c..e631957 100755
--- a/hate_crack/main.py
+++ b/hate_crack/main.py
@@ -1161,7 +1161,11 @@ def _write_field_sorted_unique(input_path, output_path, field_index, delimiter="
             open(output_path, "w") as dst,
         ):
             sort_proc = subprocess.Popen(
-                ["sort", "-u"], stdin=subprocess.PIPE, stdout=dst, text=True
+                ["sort", "-u"],
+                stdin=subprocess.PIPE,
+                stdout=dst,
+                text=True,
+                env={**os.environ, "LC_ALL": "C"},
             )
             for line in src:
                 line = line.rstrip("\n")
@@ -1542,7 +1546,10 @@ def hcatFingerprint(
             if expander_stdout is None:
                 raise RuntimeError("expander stdout pipe was not created")
             sort_proc = subprocess.Popen(
-                ["sort", "-u"], stdin=expander_stdout, stdout=dst
+                ["sort", "-u"],
+                stdin=expander_stdout,
+                stdout=dst,
+                env={**os.environ, "LC_ALL": "C"},
             )
             hcatProcess = sort_proc
             expander_stdout.close()
@@ -2993,7 +3000,10 @@ def hcatLMtoNT():
             stdout=subprocess.PIPE,
         )
         hcatProcess = subprocess.Popen(
-            ["sort", "-u"], stdin=combine_proc.stdout, stdout=combined_out
+            ["sort", "-u"],
+            stdin=combine_proc.stdout,
+            stdout=combined_out,
+            env={**os.environ, "LC_ALL": "C"},
         )
         combine_proc.stdout.close()
         try:
diff --git a/tests/test_sort_locale.py b/tests/test_sort_locale.py
new file mode 100644
index 0000000..9a3d0d3
--- /dev/null
+++ b/tests/test_sort_locale.py
@@ -0,0 +1,163 @@
+"""Sort subprocess calls must force LC_ALL=C.
+
+macOS `sort` is locale-strict: when stdin contains bytes that aren't
+valid in the current LC_COLLATE (commonly LC_COLLATE=en_US.UTF-8),
+`sort` errors out with "sort: Illegal byte sequence". Cracked-password
+streams routinely contain such bytes (hex-encoded fields, mixed
+encodings, binary), so all three `sort -u` invocations in main.py must
+override LC_ALL=C to fall back to byte-collation.
+"""
+
+import importlib
+import io
+import subprocess
+
+
+def _collect_sort_calls(monkeypatch, hc_main):
+    """Replace subprocess.Popen with a recorder. Returns a list that will
+    receive (args, kwargs) tuples for every Popen call so tests can pick
+    out the sort invocations and inspect their env."""
+    calls = []
+
+    class RecordingPopen:
+        def __init__(self, args, **kwargs):
+            calls.append((args, kwargs))
+            self.args = args
+            self.pid = 0
+            self.stdout = None
+            self.stdin = None
+            cmd0 = args[0] if args else None
+            # Pipe behavior for the sort step so callers can read/write
+            # without blocking.
+            if cmd0 == "sort":
+                stdin = kwargs.get("stdin")
+                stdout = kwargs.get("stdout")
+                data = b""
+                if stdin is not None and hasattr(stdin, "read"):
+                    data = stdin.read()
+                    if isinstance(data, str):
+                        data = data.encode()
+                if stdout is not None and hasattr(stdout, "write"):
+                    for ln in sorted(set(data.splitlines())):
+                        stdout.write(ln + b"\n" if isinstance(ln, bytes) else ln + "\n")
+                    if hasattr(stdout, "flush"):
+                        stdout.flush()
+                # Sort also receives writes via stdin.write() in
+                # _write_field_sorted_unique. For that path we need a
+                # writable stdin handle.
+                if stdin is subprocess.PIPE or kwargs.get("text"):
+                    self.stdin = io.StringIO()
+            elif isinstance(cmd0, str) and "expander" in cmd0:
+                stdin = kwargs.get("stdin")
+                data = stdin.read() if stdin is not None else b""
+                self.stdout = io.BytesIO(data)
+
+        def wait(self, timeout=None):
+            return 0
+
+        def kill(self):
+            pass
+
+    monkeypatch.setattr(hc_main.subprocess, "Popen", RecordingPopen)
+    return calls
+
+
+def _sort_calls(calls):
+    return [(args, kwargs) for args, kwargs in calls if args and args[0] == "sort"]
+
+
+def test_write_field_sorted_unique_uses_C_locale(monkeypatch, tmp_path):
+    monkeypatch.setenv("HATE_CRACK_SKIP_INIT", "1")
+    import hate_crack.main as hc_main
+
+    importlib.reload(hc_main)
+
+    src = tmp_path / "input.txt"
+    src.write_text("aa:bb\ncc:dd\n")
+    dst = tmp_path / "out.txt"
+
+    calls = _collect_sort_calls(monkeypatch, hc_main)
+    hc_main._write_field_sorted_unique(str(src), str(dst), 2)
+
+    sort_calls = _sort_calls(calls)
+    assert sort_calls, "expected at least one sort -u invocation"
+    for _args, kwargs in sort_calls:
+        env = kwargs.get("env")
+        assert env is not None, "sort Popen must pass env to force locale"
+        assert env.get("LC_ALL") == "C", (
+            f"sort env must set LC_ALL=C to handle non-UTF-8 bytes; got {env.get('LC_ALL')!r}"
+        )
+
+
+def test_hcatFingerprint_sort_uses_C_locale(monkeypatch, tmp_path):
+    monkeypatch.setenv("HATE_CRACK_SKIP_INIT", "1")
+    import hate_crack.main as hc_main
+
+    importlib.reload(hc_main)
+
+    hashfile = tmp_path / "hashes.txt"
+    out_path = tmp_path / "hashes.txt.out"
+    out_path.write_text("deadbeef:somepassword\n")
+
+    monkeypatch.setattr(hc_main, "lineCount", lambda _p: 1)
+    monkeypatch.setattr(hc_main, "hcatHashCracked", 0)
+    monkeypatch.setattr(hc_main, "ensure_binary", lambda binary_path, **_k: binary_path)
+    monkeypatch.setattr(hc_main, "hcatHybrid", lambda *a, **kw: None)
+    monkeypatch.setattr(hc_main, "hcatHashFile", str(hashfile), raising=False)
+
+    calls = _collect_sort_calls(monkeypatch, hc_main)
+    hc_main.hcatFingerprint(
+        "1000", str(hashfile), expander_len=7, run_hybrid_on_expanded=False
+    )
+
+    sort_calls = _sort_calls(calls)
+    assert sort_calls, "expected at least one sort -u invocation in fingerprint pipeline"
+    for _args, kwargs in sort_calls:
+        env = kwargs.get("env")
+        assert env is not None
+        assert env.get("LC_ALL") == "C"
+
+
+def test_all_sort_popen_calls_in_main_set_LC_ALL_C():
+    """Source-level guard: every subprocess.Popen call in main.py whose
+    first arg list begins with "sort" must also pass an env= kwarg that
+    sets LC_ALL=C. This catches the third site (hcatLMtoNT) without
+    needing to mock its substantial setup, and prevents future sort
+    invocations from regressing the locale fix."""
+    import ast
+    import pathlib
+
+    main_path = pathlib.Path(__file__).parent.parent / "hate_crack" / "main.py"
+    tree = ast.parse(main_path.read_text())
+
+    offenders = []
+    for node in ast.walk(tree):
+        if not isinstance(node, ast.Call):
+            continue
+        func = node.func
+        # Match subprocess.Popen(...)
+        if not (
+            isinstance(func, ast.Attribute)
+            and func.attr == "Popen"
+            and isinstance(func.value, ast.Name)
+            and func.value.id == "subprocess"
+        ):
+            continue
+        if not node.args:
+            continue
+        first = node.args[0]
+        # First positional arg must be a list whose first element is "sort"
+        if not isinstance(first, ast.List) or not first.elts:
+            continue
+        head = first.elts[0]
+        if not (isinstance(head, ast.Constant) and head.value == "sort"):
+            continue
+        # Found a sort Popen. Require env= kwarg.
+        env_kw = next((kw for kw in node.keywords if kw.arg == "env"), None)
+        if env_kw is None:
+            offenders.append(node.lineno)
+
+    assert not offenders, (
+        f"subprocess.Popen([\"sort\", ...]) at line(s) {offenders} must pass "
+        "env={**os.environ, 'LC_ALL': 'C'} to handle non-UTF-8 bytes on macOS."
+    )
diff --git a/tests/test_utils.py b/tests/test_utils.py
index df2f12f..fd688cf 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -191,7 +191,7 @@ def test_line_count_and_write_helpers(tmp_path, monkeypatch):
     )
 
     class FakePopen:
-        def __init__(self, args, stdin=None, stdout=None, text=None):
+        def __init__(self, args, stdin=None, stdout=None, text=None, **_kwargs):
             self.stdin = FakeStdin(self)
             self._stdout = stdout
             self._data = None