2 files changed, 4 insertions, 1 deletions
diff --git a/ext/psych/lib/psych.rb b/ext/psych/lib/psych.rb
index 31d73c1f03..8a4d63a49c 100644
--- a/ext/psych/lib/psych.rb
+++ b/ext/psych/lib/psych.rb
@@ -98,6 +98,9 @@ module Psych
   class Exception < RuntimeError
   end
 
+  class BadAlias < Exception
+  end
+
   autoload :Stream, 'psych/stream'
 
   ###
diff --git a/ext/psych/lib/psych/visitors/to_ruby.rb b/ext/psych/lib/psych/visitors/to_ruby.rb
index b8eb698754..fd899f65a6 100644
--- a/ext/psych/lib/psych/visitors/to_ruby.rb
+++ b/ext/psych/lib/psych/visitors/to_ruby.rb
@@ -201,7 +201,7 @@ module Psych
       end
 
       def visit_Psych_Nodes_Alias o
-        @st[o.anchor]
+        @st.fetch(o.anchor) { raise BadAlias, "Unknown alias: #{o.anchor}" }
       end
 
       private