diff --git a/io.h b/io.h
index a156c0e18..4406f7d4f 100644
--- a/io.h
+++ b/io.h
@@ -53,9 +53,9 @@ class io_fd_t : public io_data_t
 {
 public:
     /** fd to redirect specified fd to */
-    int old_fd;
+    const int old_fd;
     /** Whether to close old_fd */
-    int close_old;
+    const bool close_old;
 
     virtual void print() const;