diff --git a/CHANGELOG.md b/CHANGELOG.md
index b09500b..f9600ce 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -53,6 +53,17 @@
   need no changes.
   ([#25](https://github.com/tokibito/django-database-task/issues/25))
 
+### Fixed
+
+- `purge_completed_database_tasks` treated a negative `--days` like
+  `--days 0` and deleted every completed task, while
+  `purge_completed_tasks()` and the purge endpoint refuse a negative value.
+  The command now refuses it with a `CommandError` before it counts or
+  deletes anything. `--days 0`, positive values and the default are
+  unchanged, so existing projects need no changes; only an invocation with a
+  negative `--days` now fails instead of deleting the whole history.
+  ([#69](https://github.com/tokibito/django-database-task/issues/69))
+
 ## 0.6.0
 
 ### Added
diff --git a/README.md b/README.md
index c353f2c..8fd6f1c 100644
--- a/README.md
+++ b/README.md
@@ -328,7 +328,7 @@ python manage.py purge_completed_database_tasks [options]
 
 | Option | Description |
 |--------|-------------|
-| `--days` | Delete tasks completed more than N days ago (0=all) |
+| `--days` | Delete tasks completed more than N days ago (0=all). A negative value is refused |
 | `--status` | Target statuses, comma-separated (default: "SUCCESSFUL,FAILED") |
 | `--task-path` | Only delete tasks with this task path, matched exactly (default: all tasks) |
 | `--batch-size` | Number of tasks to delete at once (default: 1000) |
@@ -337,8 +337,8 @@ python manage.py purge_completed_database_tasks [options]
 The same purge is available as `purge_completed_tasks()` (see
 [Programmatic API](#programmatic-api)), for a project that purges from its own
 code. Its defaults differ from the command's: it purges the `default`
-backend's tasks only and keeps the last 7 days, and it raises `ValueError` for
-a negative `days`.
+backend's tasks only and keeps the last 7 days. Like the command, it refuses a
+negative `days`, raising `ValueError`.
 
 ### requeue_stale_database_tasks
 
diff --git a/django_database_task/management/commands/purge_completed_database_tasks.py b/django_database_task/management/commands/purge_completed_database_tasks.py
index bb92cd5..9502852 100644
--- a/django_database_task/management/commands/purge_completed_database_tasks.py
+++ b/django_database_task/management/commands/purge_completed_database_tasks.py
@@ -1,4 +1,4 @@
-from django.core.management.base import BaseCommand
+from django.core.management.base import BaseCommand, CommandError
 from django.utils.translation import gettext as _
 
 from django_database_task.executor import (
@@ -53,6 +53,9 @@ def handle(self, *args, **options):
         batch_size = options["batch_size"]
         dry_run = options["dry_run"]
 
+        if days < 0:
+            raise CommandError(f"--days must not be negative, got {days}")
+
         statuses = _parse_purge_statuses(status_str)
 
         if not statuses:
diff --git a/tests/test_commands.py b/tests/test_commands.py
index 5a4bd93..bfc4510 100644
--- a/tests/test_commands.py
+++ b/tests/test_commands.py
@@ -535,6 +535,18 @@ def test_purge_respects_days_option(self):
 
         assert DatabaseTask.objects.count() == 0
 
+    def test_purge_refuses_negative_days(self):
+        """A negative --days is refused before anything is counted or deleted."""
+        simple_task.enqueue(1, 1)
+        call_command("run_database_tasks", stdout=StringIO())
+
+        out = StringIO()
+        with pytest.raises(CommandError, match="--days must not be negative, got -1"):
+            call_command("purge_completed_database_tasks", days=-1, stdout=out)
+
+        assert DatabaseTask.objects.count() == 1
+        assert out.getvalue() == ""
+
     def test_purge_keeps_recent_tasks(self):
         """Recent tasks are not deleted."""
         simple_task.enqueue(1, 1)