diff --git a/lib/active_model/serializer/concerns/caching.rb b/lib/active_model/serializer/concerns/caching.rb
index ed893e3d..3da820e2 100644
--- a/lib/active_model/serializer/concerns/caching.rb
+++ b/lib/active_model/serializer/concerns/caching.rb
@@ -231,7 +231,7 @@ module ActiveModel
       def fetch(adapter_instance, cache_options = serializer_class._cache_options, key = nil)
         if serializer_class.cache_store
           key ||= cache_key(adapter_instance)
-          cache_options = cache_options.merge(version: object_cache_version) if object_cache_version
+          cache_options = (cache_options || {}).merge(version: object_cache_version) if object_cache_version
           serializer_class.cache_store.fetch(key, cache_options) do
             yield
           end