Use the same test for resource array-ness that ActiveModel::Serializers itself uses. This should be more reliable and it's probably good to conform to AM::Serializers behavior wherever possible.

lib/grape-active_model_serializers/formatter.rb

@@ -13,7 +13,7 @@ module Grape
           @endpoint = env["api.endpoint"]
           options   = endpoint.namespace_options.merge(endpoint.route_options)
 
-          if resource.is_a?(Array) && !resource.empty?
+          if resource.respond_to?(:to_ary) && !resource.empty?
             # ensure we have an root to fallback on
             endpoint.controller_name = resource.first.class.name.underscore.pluralize
           end