diff --git a/src/gc.c b/src/gc.c
index 4f55dc6ae..f9e6d21b9 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -1376,6 +1376,7 @@ mrb_field_write_barrier(mrb_state *mrb, struct RBasic *obj, struct RBasic *value
 
   if (!is_black(obj)) return;
   if (!is_white(value)) return;
+  if (is_red(value)) return;
 
   mrb_assert(gc->state == MRB_GC_STATE_MARK || (!is_dead(gc, value) && !is_dead(gc, obj)));
   mrb_assert(is_generational(gc) || gc->state != MRB_GC_STATE_ROOT);