diff --git a/io.h b/io.h
index 4406f7d4f..750c32072 100644
--- a/io.h
+++ b/io.h
@@ -73,7 +73,7 @@ class io_file_t : public io_data_t
     /** Filename, malloc'd. This needs to be used after fork, so don't use wcstring here. */
     const char * const filename_cstr;
     /** file creation flags to send to open */
-    int flags;
+    const int flags;
 
     virtual void print() const;