diff --git a/sklearn/exceptions.py b/sklearn/exceptions.py
index 0bd15aba1a0b0..2a99d67a4703e 100644
--- a/sklearn/exceptions.py
+++ b/sklearn/exceptions.py
@@ -114,4 +114,4 @@ class NonBLASDotWarning(EfficiencyWarning):
 
 
 class UndefinedMetricWarning(UserWarning):
-    pass
+    """Warning used when the metric is invalid"""