diff --git a/404.md b/404.md
index 0efe952b..05d15459 100644
--- a/404.md
+++ b/404.md
@@ -2,6 +2,14 @@
 layout: empty
 permalink: /404.html
 ---
+<!-- Doc latest-stable redirect -->
+<script src='/doc/versions.js'></script>
+<script>
+    if (window.location.pathname.startsWith("/doc/latest-stable/")) {
+        let real_url = window.location.href.replace("latest-stable", `v${LATEST_STABLE}`);
+        window.location.replace(real_url);
+    }
+</script>
 
 <a href="{{ '/' | relative_url }}">
 <img src="{{'/assets/images/logo-black.png' | relative_url}}" style="float:left;width:50px;height:auto;" />
diff --git a/doc/selectversion.js b/doc/selectversion.js
index 5a09070b..a7b478fe 100644
--- a/doc/selectversion.js
+++ b/doc/selectversion.js
@@ -1,7 +1,11 @@
 /// (c) Axel Naumann, CERN; 2020-03-02
+/// Later updated by giacomo.parolini@cern.ch.
+/// IMPORTANT NOTE: this script needs `versions.js` to also be included in order to work!
 'use strict';
 
 /// Configurable section.
+/// Note that some configuration is in versions.js (notably, the latest release number). It is separate because it
+/// also needs to be included in 404.html.
 
 // What the master is called. Leave untouched if master has no doc.
 const master = 'master';
@@ -28,23 +32,8 @@ function url2version(patharr) {
 
 ///=============================================================================
 
-// The latest release. NOT necessarily equal to latest-stable, as stable releases are only
-// once every two versions.
-// >>> CHANGE THIS WHEN A NEW VERSION IS RELEASED! (stable or not) <<<
-const LATEST_VERSION = 640; 
-const LATEST_VERSION_IS_STABLE = LATEST_VERSION % 4 === 0;
-const LATEST_STABLE = LATEST_VERSION - 2 * !LATEST_VERSION_IS_STABLE;
-const FIRST_VERSION = 610;
-
 // Redirect from latest-stable to the actual latest release
 let patharr = window.location.pathname.replace(/\/+/g, '/').split('/');
-if (url2version(patharr) === 'latest-stable') {
-   patharr[patharr.length - urlrootdirs] = `v${LATEST_STABLE}`;
-   let newLocation = patharr.join('/');
-   if (window.location.hash)
-      newLocation += `#${window.location.hash}`;
-   window.location.replace(newLocation);
-}
 
 let versions = ["master", "latest-stable"];
 for (let i = LATEST_VERSION; i >= FIRST_VERSION; i -= 2) {
diff --git a/doc/versions.js b/doc/versions.js
new file mode 100644
index 00000000..07ba6cfb
--- /dev/null
+++ b/doc/versions.js
@@ -0,0 +1,9 @@
+'use strict';
+
+// The latest release. NOT necessarily equal to latest-stable, as stable releases are only
+// once every two versions.
+// >>> CHANGE THIS WHEN A NEW VERSION IS RELEASED! (stable or not) <<<
+const LATEST_VERSION = 640;
+const LATEST_VERSION_IS_STABLE = LATEST_VERSION % 4 === 0;
+const LATEST_STABLE = LATEST_VERSION - 2 * !LATEST_VERSION_IS_STABLE;
+const FIRST_VERSION = 610;