diff --git a/include/ruby/internal/fl_type.h b/include/ruby/internal/fl_type.h
index 0cad5b673d..701118ef25 100644
--- a/include/ruby/internal/fl_type.h
+++ b/include/ruby/internal/fl_type.h
@@ -62,7 +62,6 @@
 #define FL_TAINT        RBIMPL_CAST((VALUE)RUBY_FL_TAINT)                /**< @old{RUBY_FL_TAINT} */
 #define FL_SHAREABLE    RBIMPL_CAST((VALUE)RUBY_FL_SHAREABLE)            /**< @old{RUBY_FL_SHAREABLE} */
 #define FL_UNTRUSTED    RBIMPL_CAST((VALUE)RUBY_FL_UNTRUSTED)            /**< @old{RUBY_FL_UNTRUSTED} */
-#define FL_SEEN_OBJ_ID  RBIMPL_CAST((VALUE)RUBY_FL_SEEN_OBJ_ID)          /**< @old{RUBY_FL_SEEN_OBJ_ID} */
 #define FL_EXIVAR       RBIMPL_CAST((VALUE)RUBY_FL_EXIVAR)               /**< @old{RUBY_FL_EXIVAR} */
 #define FL_FREEZE       RBIMPL_CAST((VALUE)RUBY_FL_FREEZE)               /**< @old{RUBY_FL_FREEZE} */
 
diff --git a/ractor.c b/ractor.c
index e5ec8d1030..928c960b8d 100644
--- a/ractor.c
+++ b/ractor.c
@@ -3598,7 +3598,6 @@ move_leave(VALUE obj, struct obj_traverse_replace_data *data)
     }
 
     // Avoid mutations using bind_call, etc.
-    // We keep FL_SEEN_OBJ_ID so GC later clean the obj_id_table.
     MEMZERO((char *)obj + sizeof(struct RBasic), char, size - sizeof(struct RBasic));
     RBASIC(obj)->flags = T_OBJECT | FL_FREEZE;
     RBASIC_SET_CLASS_RAW(obj, rb_cRactorMovedObject);