1 files changed, 7 insertions, 0 deletions
diff --git a/ext/psych/lib/psych/exception.rb b/ext/psych/lib/psych/exception.rb
index 04a9a906a4..d7469a4b30 100644
--- a/ext/psych/lib/psych/exception.rb
+++ b/ext/psych/lib/psych/exception.rb
@@ -13,6 +13,13 @@ module Psych
     end
   end
 
+  # Subclasses `BadAlias` for backwards compatibility
+  class AnchorNotDefined < BadAlias
+    def initialize anchor_name
+      super "An alias referenced an unknown anchor: #{anchor_name}"
+    end
+  end
+
   class DisallowedClass < Exception
     def initialize action, klass_name
       super "Tried to #{action} unspecified class: #{klass_name}"