diff --git a/lib/mutant/cli/classifier.rb b/lib/mutant/cli/classifier.rb
index 2839b3cb..73c98ec1 100644
--- a/lib/mutant/cli/classifier.rb
+++ b/lib/mutant/cli/classifier.rb
@@ -130,6 +130,7 @@ module Mutant
       # @api private
       #
       abstract_method :matcher
+      private         :matcher
 
     end # Classifier
   end # CLI