diff --git a/Library/Homebrew/cask/artifact/stage_only.rb b/Library/Homebrew/cask/artifact/stage_only.rb
index d0b2295c38..77ae227075 100644
--- a/Library/Homebrew/cask/artifact/stage_only.rb
+++ b/Library/Homebrew/cask/artifact/stage_only.rb
@@ -18,6 +18,10 @@ module Cask
       def to_a
         [true]
       end
+
+      def summarize
+        "true"
+      end
     end
   end
 end