diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
index eb974e4a6e..566550cbe2 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
@@ -258,7 +258,7 @@ def references(*args)
         args.each do |col|
           column("#{col}_id", :integer, options)
           column("#{col}_type", :string, polymorphic.is_a?(Hash) ? polymorphic : options) if polymorphic
-          index(polymorphic ? %w(id type).map { |t| "#{col}_#{t}" } : "#{col}_id", index_options.is_a?(Hash) ? index_options : nil) if index_options
+          index(polymorphic ? %w(id type).map { |t| "#{col}_#{t}" } : "#{col}_id", index_options.is_a?(Hash) ? index_options : {}) if index_options
         end
       end
       alias :belongs_to :references