diff --git a/src/alloc.c b/src/alloc.c
index c7f58a8adc4..c5ae8b53ddc 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -6014,7 +6014,9 @@ mark_glyph_matrix (struct glyph_matrix *matrix)
    all the references contained in it.  */
 
 #define LAST_MARKED_SIZE 500
-static Lisp_Object last_marked[LAST_MARKED_SIZE];
+/* This is not static to prevent it from being optimized away in an
+   optimized build, which then makes debugging GC problems harder.  */
+Lisp_Object last_marked[LAST_MARKED_SIZE];
 static int last_marked_index;
 
 /* For debugging--call abort when we cdr down this many