diff --git a/vm.c b/vm.c
index b76f9204ab..f26ab73dfe 100644
--- a/vm.c
+++ b/vm.c
@@ -2516,7 +2516,7 @@ rb_execution_context_mark(const rb_execution_context_t *ec)
 	rb_gc_mark_machine_stack(ec);
 	rb_gc_mark_locations((VALUE *)&ec->machine.regs,
 			     (VALUE *)(&ec->machine.regs) +
-			     sizeof(ec->machine.regs) / sizeof(VALUE));
+			     sizeof(ec->machine.regs) / (sizeof(VALUE)));
     }
 
     RUBY_MARK_UNLESS_NULL(ec->errinfo);