diff --git a/plugin.rb b/plugin.rb
index e10fd9f..fb4d9f2 100644
--- a/plugin.rb
+++ b/plugin.rb
@@ -171,7 +171,12 @@ after_initialize do
 
   on(:unassign_topic) { |topic, unassigning_user| Assigner.new(topic, unassigning_user).unassign }
 
-  Site.preloaded_category_custom_fields << "enable_unassigned_filter"
+  if respond_to?(:register_preloaded_category_custom_fields)
+    register_preloaded_category_custom_fields("enable_unassigned_filter")
+  else
+    # TODO: Drop the if-statement and this if-branch in Discourse v3.2
+    Site.preloaded_category_custom_fields << "enable_unassigned_filter"
+  end
 
   BookmarkQuery.on_preload do |bookmarks, bookmark_query|
     if SiteSetting.assign_enabled?