diff --git a/gc.c b/gc.c
index 02000815d3..ee241cbd7d 100644
--- a/gc.c
+++ b/gc.c
@@ -689,29 +689,32 @@ typedef struct RVALUE {
             VALUE v3;
         } values;
     } as;
-
-    /* Start of RVALUE_OVERHEAD.
-     * Do not directly read these members from the RVALUE as they're located
-     * at the end of the slot (which may differ in size depending on the size
-     * pool). */
-#if RACTOR_CHECK_MODE
-    uint32_t _ractor_belonging_id;
-#endif
-#if GC_DEBUG
-    const char *file;
-    int line;
-#endif
 } RVALUE;
 
-#if RACTOR_CHECK_MODE
-# define RVALUE_OVERHEAD (sizeof(RVALUE) - offsetof(RVALUE, _ractor_belonging_id))
-#elif GC_DEBUG
-# define RVALUE_OVERHEAD (sizeof(RVALUE) - offsetof(RVALUE, file))
+/* These members ae located at the end of the slot that the object is in. */
+#if RACTOR_CHECK_MODE || GC_DEBUG
+struct rvalue_overhead {
+# if RACTOR_CHECK_MODE
+    uint32_t _ractor_belonging_id;
+# endif
+# if GC_DEBUG
+    const char *file;
+    int line;
+# endif
+};
+
+// Make sure that RVALUE_OVERHEAD aligns to sizeof(VALUE)
+# define RVALUE_OVERHEAD (sizeof(struct { \
+    union { \
+        struct rvalue_overhead overhead; \
+        VALUE value; \
+    }; \
+}))
 #else
 # define RVALUE_OVERHEAD 0
 #endif
 
-STATIC_ASSERT(sizeof_rvalue, sizeof(RVALUE) == (SIZEOF_VALUE * 5) + RVALUE_OVERHEAD);
+STATIC_ASSERT(sizeof_rvalue, sizeof(RVALUE) == (SIZEOF_VALUE * 5));
 STATIC_ASSERT(alignof_rvalue, RUBY_ALIGNOF(RVALUE) == SIZEOF_VALUE);
 
 typedef uintptr_t bits_t;
@@ -955,7 +958,7 @@ typedef struct rb_objspace {
 #define HEAP_PAGE_ALIGN_LOG 16
 #endif
 
-#define BASE_SLOT_SIZE sizeof(RVALUE)
+#define BASE_SLOT_SIZE (sizeof(RVALUE) + RVALUE_OVERHEAD)
 
 #define CEILDIV(i, mod) roomof(i, mod)
 enum {
@@ -3476,7 +3479,7 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
 }
 
 
-#define OBJ_ID_INCREMENT (sizeof(RVALUE))
+#define OBJ_ID_INCREMENT (BASE_SLOT_SIZE)
 #define OBJ_ID_INITIAL (OBJ_ID_INCREMENT)
 
 static int
@@ -8194,7 +8197,7 @@ gc_compact_plane(rb_objspace_t *objspace, rb_size_pool_t *size_pool, rb_heap_t *
 
     do {
         VALUE vp = (VALUE)p;
-        GC_ASSERT(vp % sizeof(RVALUE) == 0);
+        GC_ASSERT(vp % BASE_SLOT_SIZE == 0);
 
         if (bitset & 1) {
             objspace->rcompactor.considered_count_table[BUILTIN_TYPE(vp)]++;
@@ -13652,7 +13655,7 @@ Init_GC(void)
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("DEBUG")), RBOOL(GC_DEBUG));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("BASE_SLOT_SIZE")), SIZET2NUM(BASE_SLOT_SIZE - RVALUE_OVERHEAD));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_OVERHEAD")), SIZET2NUM(RVALUE_OVERHEAD));
-    rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_SIZE")), SIZET2NUM(sizeof(RVALUE)));
+    rb_hash_aset(gc_constants, ID2SYM(rb_intern("RVALUE_SIZE")), SIZET2NUM(BASE_SLOT_SIZE));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("HEAP_PAGE_OBJ_LIMIT")), SIZET2NUM(HEAP_PAGE_OBJ_LIMIT));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("HEAP_PAGE_BITMAP_SIZE")), SIZET2NUM(HEAP_PAGE_BITMAP_SIZE));
     rb_hash_aset(gc_constants, ID2SYM(rb_intern("HEAP_PAGE_SIZE")), SIZET2NUM(HEAP_PAGE_SIZE));