diff --git a/activemodel/lib/active_model/naming.rb b/activemodel/lib/active_model/naming.rb
index b880835b41..7022146e63 100644
--- a/activemodel/lib/active_model/naming.rb
+++ b/activemodel/lib/active_model/naming.rb
@@ -171,6 +171,7 @@ module ActiveModel
       @klass        = klass
       @singular     = _singularize(@name)
       @plural       = ActiveSupport::Inflector.pluralize(@singular, locale)
+      @uncountable  = @plural == @singular
       @element      = ActiveSupport::Inflector.underscore(ActiveSupport::Inflector.demodulize(@name))
       @human        = ActiveSupport::Inflector.humanize(@element)
       @collection   = ActiveSupport::Inflector.tableize(@name)
@@ -179,7 +180,7 @@ module ActiveModel
 
       @route_key          = (namespace ? ActiveSupport::Inflector.pluralize(@param_key, locale) : @plural.dup)
       @singular_route_key = ActiveSupport::Inflector.singularize(@route_key, locale)
-      @route_key << "_index" if @plural == @singular
+      @route_key << "_index" if @uncountable
     end
 
     # Transform the model name into a more human format, using I18n. By default,
@@ -207,6 +208,10 @@ module ActiveModel
       I18n.translate(defaults.shift, **options)
     end
 
+    def uncountable?
+      @uncountable
+    end
+
     private
       def _singularize(string)
         ActiveSupport::Inflector.underscore(string).tr("/", "_")
@@ -280,7 +285,7 @@ module ActiveModel
     #   ActiveModel::Naming.uncountable?(Sheep) # => true
     #   ActiveModel::Naming.uncountable?(Post)  # => false
     def self.uncountable?(record_or_class)
-      plural(record_or_class) == singular(record_or_class)
+      model_name_from_record_or_class(record_or_class).uncountable?
     end
 
     # Returns string to use while generating route names. It differs for
diff --git a/activemodel/test/cases/naming_test.rb b/activemodel/test/cases/naming_test.rb
index 16dcadc463..51fbc4cf30 100644
--- a/activemodel/test/cases/naming_test.rb
+++ b/activemodel/test/cases/naming_test.rb
@@ -42,6 +42,10 @@ class NamingTest < ActiveModel::TestCase
   def test_i18n_key
     assert_equal :"post/track_back", @model_name.i18n_key
   end
+
+  def test_uncountable
+    assert_equal false, @model_name.uncountable?
+  end
 end
 
 class NamingWithNamespacedModelInIsolatedNamespaceTest < ActiveModel::TestCase