diff --git a/app/controllers/work_packages/auto_completes_controller.rb b/app/controllers/work_packages/auto_completes_controller.rb
index 7fd1ef456b2..db5be40c50d 100644
--- a/app/controllers/work_packages/auto_completes_controller.rb
+++ b/app/controllers/work_packages/auto_completes_controller.rb
@@ -60,10 +60,11 @@ class WorkPackages::AutoCompletesController < ApplicationController
       # `displayId` collapses to the numeric id in classic mode and to the
       # semantic identifier in semantic mode. The CKEditor mention plugin
       # reads it to insert `#PROJ-7` (or `#1234`) into the markdown source
-      # and to build the mention's link URL.
+      # and to build the mention's link URL. Stringified for parity with
+      # APIv3 (`WorkPackageRepresenter` serialises `displayId` as a string).
       work_package.attributes.merge(
         "to_s" => work_package.to_s,
-        "displayId" => work_package.display_id
+        "displayId" => work_package.display_id.to_s
       )
     end
   end