diff --git a/tests/test_thinner.py b/tests/test_thinner.py
index 8d9c84b..3de2120 100644
--- a/tests/test_thinner.py
+++ b/tests/test_thinner.py
@@ -36,6 +36,9 @@ class TestThinner(unittest2.TestCase):
         with self.assertRaisesRegexp(Exception, "^Invalid schedule"):
             ThinnerRule("XXX")
 
+        with self.assertRaisesRegexp(Exception, "^Number of"):
+            Thinner("-1")
+
 
     def test_incremental(self):
         ok=['2023-01-03 10:53:16',
diff --git a/zfs_autobackup/Thinner.py b/zfs_autobackup/Thinner.py
index 9194a5a..232b2d3 100644
--- a/zfs_autobackup/Thinner.py
+++ b/zfs_autobackup/Thinner.py
@@ -19,7 +19,7 @@ class Thinner:
 
         rule_strs = schedule_str.split(",")
         for rule_str in rule_strs:
-            if rule_str.isdigit():
+            if rule_str.lstrip('-').isdigit():
                 self.always_keep = int(rule_str)
                 if self.always_keep < 0:
                     raise (Exception("Number of snapshots to keep cant be negative: {}".format(self.always_keep)))