diff --git a/io.c b/io.c
index 33350ea752..eccd429352 100644
--- a/io.c
+++ b/io.c
@@ -201,7 +201,7 @@ rb_update_max_fd(int fd)
     rb_atomic_t max_fd = max_file_descriptor;
     int err;
 
-    if (afd <= max_fd)
+    if (fd < 0 || afd <= max_fd)
         return;
 
 #if defined(HAVE_FCNTL) && defined(F_GETFL)