Rails – Ajax: ActionController::UnknownFormat

I am trying to build an ecommerce site using RoR. I have this logic where the user can add more or less quantity to the product they would like to purchase:

<%= link_to order_item_path(item, 'minus'), remote: true, method: :patch do %>
  <button>-</button>
<%end %>
<%= item.quantity %>
<%= link_to order_item_path(item, 'plus'), remote: true, method: :patch do %>
 <button>+</button>
<%end %>

So every time they click on plus or minus, a patch call is sent to the OrderItems controller:

def update
 #do some logic
 respond_to do |format|
   format.js 
   format.html { redirect_to new_charge_path }
 end
end

In the controller we do some logic and I don´t want the page to refresh but to use JavaScript to make the number go up or down. I want to do that using js.erb:

#order_items/update.js.erb

console.log("plus minus") 
#here make number go up or down

The problem is that when the user clicks the button of plus or minus, I get an error:

ActionController::UnknownFormat in OrderItemsController#update
respond_to do |format|
 format.js 
 format.html { redirect_to new_charge_path }
end

Anyone knows why it is happening and how could I solve it? Thanks!