diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6b38d9009d..8cb4fc5f9a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,5 @@
+*   Fix railtie logic that adds Active Job serialization support.
+
 ## Active Resource 5.1.0 (Nov 2, 2018) ##
 
 *   Improve support of Active Resource objects inside fibers.
diff --git a/lib/active_resource/railtie.rb b/lib/active_resource/railtie.rb
index b30bb06148..fb6762d131 100644
--- a/lib/active_resource/railtie.rb
+++ b/lib/active_resource/railtie.rb
@@ -16,7 +16,7 @@ class Railtie < Rails::Railtie
     end
 
     initializer "active_resource.add_active_job_serializer" do |app|
-      if defined? app.config.active_job.custom_serializers
+      if app.config.try(:active_job)&.custom_serializers
         require "active_resource/active_job_serializer"
         app.config.active_job.custom_serializers << ActiveResource::ActiveJobSerializer
       end