diff --git a/lib/active_model/serializer/adapter/fragment_cache.rb b/lib/active_model/serializer/adapter/fragment_cache.rb
index 231ddaed..ae15995f 100644
--- a/lib/active_model/serializer/adapter/fragment_cache.rb
+++ b/lib/active_model/serializer/adapter/fragment_cache.rb
@@ -54,8 +54,8 @@ module ActiveModel
         end
 
         def fragment_serializer(name, klass)
-          cached     = "#{name.capitalize}CachedSerializer"
-          non_cached = "#{name.capitalize}NonCachedSerializer"
+          cached     = "#{to_valid_const_name(name)}CachedSerializer"
+          non_cached = "#{to_valid_const_name(name)}NonCachedSerializer"
 
           Object.const_set cached, Class.new(ActiveModel::Serializer) unless Object.const_defined?(cached)
           Object.const_set non_cached, Class.new(ActiveModel::Serializer) unless Object.const_defined?(non_cached)
@@ -72,6 +72,10 @@ module ActiveModel
           cached_attributes(klass, serializers)
           serializers
         end
+
+        def to_valid_const_name(name)
+          name.gsub('::', '_')
+        end
       end
     end
   end
diff --git a/test/adapter/fragment_cache_test.rb b/test/adapter/fragment_cache_test.rb
index d249637a..e2e4e2f4 100644
--- a/test/adapter/fragment_cache_test.rb
+++ b/test/adapter/fragment_cache_test.rb
@@ -4,11 +4,14 @@ module ActiveModel
     class Adapter
       class FragmentCacheTest < Minitest::Test
         def setup
+          @spam            = Spam::UnrelatedLink.new(id: "spam-id-1")
           @author          = Author.new(name: 'Joao M. D. Moura')
           @role            = Role.new(name: 'Great Author', description:nil)
           @role.author     = [@author]
           @role_serializer = RoleSerializer.new(@role)
+          @spam_serializer = Spam::UnrelatedLinkSerializer.new(@spam)
           @role_hash       = FragmentCache.new(RoleSerializer.adapter.new(@role_serializer), @role_serializer, {})
+          @spam_hash       = FragmentCache.new(Spam::UnrelatedLinkSerializer.adapter.new(@spam_serializer), @spam_serializer, {})
         end
 
         def test_fragment_fetch_with_virtual_attributes
@@ -20,6 +23,13 @@ module ActiveModel
           }
           assert_equal(@role_hash.fetch, expected_result)
         end
+
+        def test_fragment_fetch_with_namespaced_object
+          expected_result = {
+            id: @spam.id
+          }
+          assert_equal(@spam_hash.fetch, expected_result)
+        end
       end
     end
   end
diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index 1a52dcec..26397191 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -250,6 +250,7 @@ VirtualValueSerializer = Class.new(ActiveModel::Serializer) do
 end
 
 Spam::UnrelatedLinkSerializer = Class.new(ActiveModel::Serializer) do
+  cache only: [:id]
   attributes :id
 end