diff --git a/include/class.thread.php b/include/class.thread.php
index 80b094ed4fd99ab3b6560c2f9e03089b5329a867..147712324f1e46fe4a4ab77ccedb8125dc3b8f9f 100644
--- a/include/class.thread.php
+++ b/include/class.thread.php
@@ -370,7 +370,10 @@ Class ThreadEntry {
     }
 
     function getEmailReferencesForUser($user) {
-        return $this->getTaggedEmailReferences('u', $user->getId());
+        return $this->getTaggedEmailReferences('u',
+            ($user instanceof Collaborator)
+                ? $user->getUserId()
+                : $user->getId());
     }
 
     function getEmailReferencesForStaff($staff) {