diff --git a/gc/default.c b/gc/default.c
index d9f6623f66013a..1dcc9907ca829d 100644
--- a/gc/default.c
+++ b/gc/default.c
@@ -2533,7 +2533,7 @@ rb_gc_impl_obj_slot_size(VALUE obj)
 #if USE_MMTK
     if (rb_mmtk_enabled_p()) {
         // Load from our hidden field before the object.
-        return *(size_t*)(obj - MMTK_OBJREF_OFFSET);
+        return rb_mmtk_get_payload_size(obj);
     }
 #endif
 
@@ -3290,7 +3290,8 @@ rb_mmtk_each_objects_safe(each_obj_callback *callback, void *data)
     // If GC is triggered in `callback`, `tmpbuf` will keep elements of `array` alive.
     for (size_t i = 0; i < build_array_data.len; i++) {
         volatile VALUE object = array[i];
-        size_t object_size = rb_mmtk_get_object_size(object);
+        // This is the "object size" seen by the callback.  It's only the payload size.
+        size_t object_size = rb_mmtk_get_payload_size(object);
         uintptr_t object_end = object + object_size;
 
         RUBY_DEBUG_LOG("Enumerating object: %p\n", (void*)object);
diff --git a/internal/mmtk.h b/internal/mmtk.h
index 6231694c8f99fa..1f58042365d5d1 100644
--- a/internal/mmtk.h
+++ b/internal/mmtk.h
@@ -29,6 +29,10 @@ typedef uint32_t MMTk_AllocationSemantics;
 
 #define MMTK_GC_THREAD_KIND_WORKER 1
 
+#define MMTK_HAS_MOVED_GIVTBL 9223372036854775808ull
+
+#define MMTK_HIDDEN_SIZE_MASK 281474976710655
+
 typedef struct st_table st_table;
 
 typedef struct RubyBindingOptions {
@@ -112,6 +116,10 @@ typedef struct MMTk_RawVecOfObjRef {
     size_t capa;
 } MMTk_RawVecOfObjRef;
 
+typedef struct MMTk_HiddenHeader {
+    size_t prefix;
+} MMTk_HiddenHeader;
+
 /**
  * Create an MMTKBuilder instance with default options.
  * This instance shall be consumed by `mmtk_init_binding`.
diff --git a/internal/mmtk_support.h b/internal/mmtk_support.h
index bca8c2b72c2052..5977806ab84869 100644
--- a/internal/mmtk_support.h
+++ b/internal/mmtk_support.h
@@ -49,7 +49,8 @@ void rb_mmtk_destroy_mutator(MMTk_VMMutatorThread cur_thread, bool at_fork);
 // Object layout
 size_t rb_mmtk_prefix_size(void);
 size_t rb_mmtk_suffix_size(void);
-size_t rb_mmtk_get_object_size(VALUE object);
+void rb_mmtk_init_hidden_header(VALUE object, size_t payload_size);
+size_t rb_mmtk_get_payload_size(VALUE object);
 
 // Allocation
 VALUE rb_mmtk_alloc_obj(size_t mmtk_alloc_size, size_t size_pool_size, size_t prefix_size);
diff --git a/mmtk_support.c b/mmtk_support.c
index abd6197ee8b1ed..e060f078909be1 100644
--- a/mmtk_support.c
+++ b/mmtk_support.c
@@ -395,12 +395,27 @@ rb_mmtk_suffix_size(void)
     return ruby_binding_options.suffix_size;
 }
 
-size_t
-rb_mmtk_get_object_size(VALUE object)
+void
+rb_mmtk_init_hidden_header(VALUE object, size_t payload_size)
 {
-    return *(size_t*)(object - sizeof(VALUE));
+    RUBY_ASSERT(payload_size <= MMTK_HIDDEN_SIZE_MASK,
+                "payload size greater than MMTK_HIDDEN_SIZE_MASK. payload_size: %zu", payload_size);
+
+    struct MMTk_HiddenHeader *hidden_header = (struct MMTk_HiddenHeader*)(object - MMTK_OBJREF_OFFSET);
+    hidden_header->prefix = payload_size;
 }
 
+size_t
+rb_mmtk_get_payload_size(VALUE object)
+{
+    struct MMTk_HiddenHeader *hidden_header = (struct MMTk_HiddenHeader*)(object - MMTK_OBJREF_OFFSET);
+    size_t prefix = hidden_header->prefix;
+
+    RUBY_ASSERT((prefix & ~(MMTK_HIDDEN_SIZE_MASK | MMTK_HAS_MOVED_GIVTBL)) == 0,
+                "Hidden field is corrupted.  Object: %p, prefix: %zx", (void*) object, prefix);
+
+    return prefix & MMTK_HIDDEN_SIZE_MASK;
+}
 
 ////////////////////////////////////////////////////////////////////////////////
 // Allocation
@@ -495,13 +510,13 @@ rb_mmtk_alloc_obj(size_t mmtk_alloc_size, size_t size_pool_size, size_t prefix_s
     // Allocate the object.
     void *addr = rb_mmtk_alloc(mmtk_alloc_size, semantics);
 
-    // Store the Ruby-level object size before the object.
-    *(size_t*)addr = size_pool_size;
-
     // The Ruby-level object reference (i.e. VALUE) is at an offset from the MMTk-level
     // allocation unit.
     VALUE obj = (VALUE)addr + prefix_size;
 
+    // Store the Ruby-level object size before the object.
+    rb_mmtk_init_hidden_header(obj, size_pool_size);
+
     rb_mmtk_post_alloc(obj, mmtk_alloc_size, semantics);
 
     return obj;