diff --git a/checks/__init__.py b/checks/__init__.py
index 248af288a5..1f08da47b4 100644
--- a/checks/__init__.py
+++ b/checks/__init__.py
@@ -130,10 +130,16 @@ def _rate(cls, sample1, sample2):
             interval = sample2[0] - sample1[0]
             if interval == 0:
                 raise Infinity()
+ 
             delta = sample2[1] - sample1[1]
+            if delta < 0:
+                raise UnknownValue()
+
             return (sample2[0], delta / interval)
         except Infinity:
             raise
+        except UnknownValue:
+            raise
         except Exception, e:
             raise NaN(e)
 
diff --git a/tests/test_common.py b/tests/test_common.py
index e3f95ce715..6569547481 100644
--- a/tests/test_common.py
+++ b/tests/test_common.py
@@ -42,17 +42,17 @@ def test_counter(self):
         self.assertEquals(self.c.get_sample("test-counter"), 1.0)
         self.assertEquals(self.c.get_sample_with_timestamp("test-counter"), (2.0, 1.0))
         self.c.save_sample("test-counter", -2.0, 3.0)
-        self.assertEquals(self.c.get_sample_with_timestamp("test-counter"), (3.0, -4.0))
+        self.assertRaises(UnknownValue, self.c.get_sample_with_timestamp, "test-counter")
 
     def test_samples(self):
         self.assertEquals(self.c.get_samples(), {})
         self.c.save_sample("test-metric", 1.0, 0.0)  # value, ts
         self.c.save_sample("test-counter", 1.0, 1.0) # value, ts
-        self.c.save_sample("test-counter", 0.0, 2.0) # value, ts
+        self.c.save_sample("test-counter", 4.0, 2.0) # value, ts
         assert "test-metric"  in self.c.get_samples_with_timestamps(), self.c.get_samples_with_timestamps()
         self.assertEquals(self.c.get_samples_with_timestamps()["test-metric"], (0.0, 1.0))
         assert "test-counter" in self.c.get_samples_with_timestamps(), self.c.get_samples_with_timestamps()
-        self.assertEquals(self.c.get_samples_with_timestamps()["test-counter"], (2.0, -1.0))
+        self.assertEquals(self.c.get_samples_with_timestamps()["test-counter"], (2.0, 3.0))
 
 if __name__ == '__main__':
     unittest.main()