diff --git a/gc/mmtk.c b/gc/mmtk.c
index b5a46f7ef8965e..4850853ef5118e 100644
--- a/gc/mmtk.c
+++ b/gc/mmtk.c
@@ -528,6 +528,7 @@ rb_gc_impl_init(void)
 {
     VALUE gc_constants = rb_hash_new();
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("BASE_SLOT_SIZE")), SIZET2NUM(sizeof(VALUE) * 5));
+    rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_OVERHEAD")), INT2NUM(0));
     // Pretend we have 5 size pools
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("SIZE_POOL_COUNT")), LONG2FIX(5));
     OBJ_FREEZE(gc_constants);