diff --git a/src/lisp.h b/src/lisp.h
index 871667cf0ab..5ccc4b3dc4b 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -1879,6 +1879,7 @@ extern int consing_since_gc;
 /* Threshold for doing another gc.  */
 
 extern EMACS_INT gc_cons_threshold;
+extern EMACS_INT gc_cons_combined_threshold;
 
 /* Structure for recording stack slots that need marking.  */