diff --git a/lisp/gnus/gnus-sum.el b/lisp/gnus/gnus-sum.el
index f09c0fbb58e..f5853a24305 100644
--- a/lisp/gnus/gnus-sum.el
+++ b/lisp/gnus/gnus-sum.el
@@ -4750,7 +4750,8 @@ If LINE, insert the rebuilt thread starting on line LINE."
 
 (defun gnus-id-to-thread (id)
   "Return the (sub-)thread where ID appears."
-  (gethash id gnus-newsgroup-dependencies))
+  (when (hash-table-p gnus-newsgroup-dependencies)
+    (gethash id gnus-newsgroup-dependencies)))
 
 (defun gnus-id-to-article (id)
   "Return the article number of ID."