diff --git a/iseq.c b/iseq.c
index 19f475b0970658..49f2765551db12 100644
--- a/iseq.c
+++ b/iseq.c
@@ -159,6 +159,14 @@ iseq_extract_values(const VALUE *code, size_t pos, iseq_value_itr_t * func, void
 		    }
 		    break;
 		}
+	    case TS_IC:
+		if (BIN(once) == insn || BIN(trace_once) == insn) {
+		    union iseq_inline_storage_entry *const is = (union iseq_inline_storage_entry *)code[pos + op_no + 1];
+		    if (is->once.value) {
+			func(data, is->once.value);
+		    }
+		}
+		break;
 	    default:
 		break;
 	}
@@ -397,13 +405,6 @@ set_relation(rb_iseq_t *iseq, const rb_iseq_t *piseq)
     }
 }
 
-void
-rb_iseq_add_mark_object(const rb_iseq_t *iseq, VALUE obj)
-{
-    /* TODO: check dedup */
-    rb_ary_push(ISEQ_MARK_ARY(iseq), obj);
-}
-
 static VALUE
 prepare_iseq_build(rb_iseq_t *iseq,
 		   VALUE name, VALUE path, VALUE realpath, VALUE first_lineno, const rb_code_location_t *code_location,
diff --git a/iseq.h b/iseq.h
index f8578bf522d6af..0dd0703d207054 100644
--- a/iseq.h
+++ b/iseq.h
@@ -174,7 +174,6 @@ void rb_iseq_build_from_ary(rb_iseq_t *iseq, VALUE misc,
 			    VALUE exception, VALUE body);
 
 /* iseq.c */
-void rb_iseq_add_mark_object(const rb_iseq_t *iseq, VALUE obj);
 VALUE rb_iseq_load(VALUE data, VALUE parent, VALUE opt);
 VALUE rb_iseq_parameters(const rb_iseq_t *iseq, int is_proc);
 struct st_table *ruby_insn_make_insn_table(void);
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index acb7d7999d7770..22c179e6a667f4 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -3294,10 +3294,10 @@ vm_once_dispatch(rb_execution_context_t *ec, ISEQ iseq, IC ic)
     else if (is->once.running_thread == NULL) {
 	VALUE val;
 	is->once.running_thread = th;
-	val = is->once.value = rb_ensure(vm_once_exec, (VALUE)iseq, vm_once_clear, (VALUE)is);
+	val = rb_ensure(vm_once_exec, (VALUE)iseq, vm_once_clear, (VALUE)is);
+	RB_OBJ_WRITE(ec->cfp->iseq, &is->once.value, val);
 	/* is->once.running_thread is cleared by vm_once_clear() */
 	is->once.running_thread = RUNNING_THREAD_ONCE_DONE; /* success */
-	rb_iseq_add_mark_object(ec->cfp->iseq, val);
 	return val;
     }
     else if (is->once.running_thread == th) {