diff --git a/ctapipe/reco/sklearn.py b/ctapipe/reco/sklearn.py
index bb8e9b8af0c..f81e6d2cf7b 100644
--- a/ctapipe/reco/sklearn.py
+++ b/ctapipe/reco/sklearn.py
@@ -307,7 +307,8 @@ class SKLearnClassificationReconstructor(SKLearnReconstructor):
     ).tag(config=True)
 
     invalid_class = traits.Integer(
-        default_value=-1, help="The label to fill in case no prediction could be made."
+        default_value=-1,
+        help="The label value to fill in case no prediction could be made.",
     ).tag(config=True)
 
     positive_class = traits.Integer(