diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index a89a79a8e..e5a56cd5a 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -122,7 +122,11 @@ def build_serializer_class(resource, options)
         "".tap do |klass_name|
           klass_name << "#{options[:namespace]}::" if options[:namespace]
           klass_name << options[:prefix].to_s.classify if options[:prefix]
-          klass_name << "#{resource.class.name}Serializer"
+          if resource.is_a?(String)
+            klass_name << "#{resource}Serializer"
+          else
+            klass_name << "#{resource.class.name}Serializer"
+          end
         end
       end
 
diff --git a/test/unit/active_model/array_serializer/serialization_test.rb b/test/unit/active_model/array_serializer/serialization_test.rb
index 890e7a09a..92df3d1e0 100644
--- a/test/unit/active_model/array_serializer/serialization_test.rb
+++ b/test/unit/active_model/array_serializer/serialization_test.rb
@@ -49,6 +49,7 @@ def test_serializer_for_array_returns_appropriate_type
         def object.serializer_class; CustomSerializer; end
 
         assert_equal CustomSerializer, Serializer.serializer_for(object)
+        assert_equal CustomSerializer, Serializer.serializer_for('Custom')
       end
     end