diff --git a/insns.def b/insns.def
index eb5b4dec9e..679927b4e9 100644
--- a/insns.def
+++ b/insns.def
@@ -1250,7 +1250,7 @@ once
 	}
 	else if (is->once.running_thread == th) {
 	    /* recursive once */
-	    val = vm_once_exec(iseq);
+	    val = vm_once_exec((VALUE)iseq);
 	}
 	else {
 	    /* waiting for finish */
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 2a6ad6deeb..64e58f0842 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -2397,9 +2397,9 @@ vm_make_proc_with_iseq(rb_iseq_t *blockiseq)
 }
 
 static VALUE
-vm_once_exec(rb_iseq_t *iseq)
+vm_once_exec(VALUE iseq)
 {
-    VALUE proc = vm_make_proc_with_iseq(iseq);
+    VALUE proc = vm_make_proc_with_iseq((rb_iseq_t *)iseq);
     return rb_proc_call_with_block(proc, 0, 0, Qnil);
 }