How to include polymorphic association in 'includes' Rails

Question

I have this in model:

    belongs_to :merchant_owner, :polymorphic => true

I want to make query like this:

   @search = Merchant.includes(:merchant_owner).search
   @merchants = @search.order

But I get this error in views:

 You can't create a polymorphic belongs_to join without specifying the polymorphic class!

for this line:

@search.count

I am using ruby 1.8.7 and rails 3.0.20 and meta search 1.0.6. Does anyone know how to make the includes work with merchant_owner?