diff --git a/lib/active_model/serializable.rb b/lib/active_model/serializable.rb
index 344d8e4e..d23aae7d 100644
--- a/lib/active_model/serializable.rb
+++ b/lib/active_model/serializable.rb
@@ -33,7 +33,11 @@ module ActiveModel
     end
 
     def namespace
-      get_namespace && Utils._const_get(get_namespace)
+      if module_name = get_namespace
+        Serializer.serializers_cache.fetch_or_store(module_name) do
+          Utils._const_get(module_name)
+        end
+      end
     end
 
     def embedded_in_root_associations
diff --git a/lib/active_model/serializable/utils.rb b/lib/active_model/serializable/utils.rb
index 3b5f92c3..efabb6c4 100644
--- a/lib/active_model/serializable/utils.rb
+++ b/lib/active_model/serializable/utils.rb
@@ -4,15 +4,13 @@ module ActiveModel
       extend self
 
       def _const_get(const)
-        Serializer.serializers_cache.fetch_or_store(const) do
-          begin
-            method = RUBY_VERSION >= '2.0' ? :const_get : :qualified_const_get
-            Object.send method, const
-          rescue NameError
-            const.safe_constantize
-          end
+        begin
+          method = RUBY_VERSION >= '2.0' ? :const_get : :qualified_const_get
+          Object.send method, const
+        rescue NameError
+          const.safe_constantize
         end
       end
     end
   end
-end
+end
\ No newline at end of file
diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index b5ff05da..a89a79a8 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -66,7 +66,10 @@ end
             ArraySerializer
           end
         else
-          _const_get build_serializer_class(resource, options)
+          klass_name = build_serializer_class(resource, options)
+          Serializer.serializers_cache.fetch_or_store(klass_name) do
+            _const_get(klass_name)
+          end
         end
       end