diff --git a/class.c b/class.c
index f5090d7f0f..01939f5008 100644
--- a/class.c
+++ b/class.c
@@ -177,9 +177,7 @@ class_alloc(VALUE flags, VALUE klass)
 
     RVARGC_NEWOBJ_OF(obj, struct RClass, klass, (flags & T_MASK) | FL_PROMOTED1 /* start from age == 2 */ | (RGENGC_WB_PROTECTED_CLASS ? FL_WB_PROTECTED : 0), alloc_size);
 
-#if USE_RVARGC
-    obj->ptr = (rb_classext_t *)((char *)obj + sizeof(struct RClass));
-#else
+#if !USE_RVARGC
     obj->ptr = ZALLOC(rb_classext_t);
 #endif
 
diff --git a/gc.c b/gc.c
index 94c7f60c95..2c64460541 100644
--- a/gc.c
+++ b/gc.c
@@ -4588,11 +4588,11 @@ obj_memsize_of(VALUE obj, int use_all_types)
                 // TODO: more correct value
 		size += st_memsize(RCLASS_IV_INDEX_TBL(obj));
 	    }
-	    if (RCLASS(obj)->ptr->iv_tbl) {
-		size += st_memsize(RCLASS(obj)->ptr->iv_tbl);
+            if (RCLASS_EXT(obj)->iv_tbl) {
+                size += st_memsize(RCLASS_EXT(obj)->iv_tbl);
 	    }
-	    if (RCLASS(obj)->ptr->const_tbl) {
-		size += rb_id_table_memsize(RCLASS(obj)->ptr->const_tbl);
+            if (RCLASS_EXT(obj)->const_tbl) {
+                size += rb_id_table_memsize(RCLASS_EXT(obj)->const_tbl);
 	    }
             if (RCLASS_CC_TBL(obj)) {
                 size += cc_table_memsize(RCLASS_CC_TBL(obj));
diff --git a/internal/class.h b/internal/class.h
index a949bfb3e6..ee36ad1967 100644
--- a/internal/class.h
+++ b/internal/class.h
@@ -66,7 +66,9 @@ struct rb_classext_struct {
 struct RClass {
     struct RBasic basic;
     VALUE super;
+#if !USE_RVARGC
     struct rb_classext_struct *ptr;
+#endif
 #if SIZEOF_SERIAL_T == SIZEOF_VALUE
     /* Class serial is as wide as VALUE.  Place it here. */
     rb_serial_t class_serial;
@@ -79,7 +81,11 @@ struct RClass {
 typedef struct rb_subclass_entry rb_subclass_entry_t;
 typedef struct rb_classext_struct rb_classext_t;
 
-#define RCLASS_EXT(c) (RCLASS(c)->ptr)
+#if USE_RVARGC
+#  define RCLASS_EXT(c) ((rb_classext_t *)((char *)c + sizeof(struct RClass)))
+#else
+#  define RCLASS_EXT(c) (RCLASS(c)->ptr)
+#endif
 #define RCLASS_IV_TBL(c) (RCLASS_EXT(c)->iv_tbl)
 #define RCLASS_CONST_TBL(c) (RCLASS_EXT(c)->const_tbl)
 #if SIZEOF_SERIAL_T == SIZEOF_VALUE