diff --git a/lib/ransack/context.rb b/lib/ransack/context.rb
index dbb99666f..0d0c72a3b 100644
--- a/lib/ransack/context.rb
+++ b/lib/ransack/context.rb
@@ -152,6 +152,7 @@ def unpolymorphize_association(str)
     end
 
     def ransackable_alias(str)
+      return nil if str.nil?
       klass._ransack_aliases.fetch(str, klass._ransack_aliases.fetch(str.to_sym, str))
     end