diff --git a/src/gc.c b/src/gc.c
index d9eb18280..4b6efdc39 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -1048,6 +1048,7 @@ incremental_marking_phase(mrb_state *mrb, mrb_gc *gc, size_t limit)
   while (gc->gray_list && tried_marks < limit) {
     struct RBasic *obj = gc->gray_list;
     gc->gray_list = obj->gcnext;
+    obj->gcnext = NULL;
     gc_mark_children(mrb, gc, obj);
     tried_marks += gc_gray_counts(mrb, gc, obj);
   }