diff --git a/src/Composer/Autoload/AutoloadGenerator.php b/src/Composer/Autoload/AutoloadGenerator.php
index 474efcbd5..6a8848976 100644
--- a/src/Composer/Autoload/AutoloadGenerator.php
+++ b/src/Composer/Autoload/AutoloadGenerator.php
@@ -145,19 +145,23 @@ class AutoloadGenerator
             // Force scanPsrPackages when classmap is authoritative
             $scanPsrPackages = true;
         }
-        if ($this->runScripts) {
+
+        // auto-set devMode based on whether dev dependencies are installed or not
+        if (null === $this->devMode) {
+            // we assume no-dev mode if no vendor dir is present or it is too old to contain dev information
+            $this->devMode = false;
+
             $installedJson = new JsonFile($config->get('vendor-dir').'/composer/installed.json');
-            $isDevInstall = null;
             if ($installedJson->exists()) {
                 $installedJson = $installedJson->read();
-                $isDevInstall = isset($installedJson['dev']) ? $installedJson['dev'] : null;
-            }
-            // auto-set devMode based on whether dev dependencies are installed or not
-            if (null !== $isDevInstall && null === $this->devMode) {
-                $this->devMode = $isDevInstall;
+                if (isset($installedJson['dev'])) {
+                    $this->devMode = $installedJson['dev'];
+                }
             }
+        }
 
-            // set COMPOSER_DEV_MODE in case not set yet so it is available in the dump-autoload autoload
+        if ($this->runScripts) {
+            // set COMPOSER_DEV_MODE in case not set yet so it is available in the dump-autoload event listeners
             if (!isset($_SERVER['COMPOSER_DEV_MODE'])) {
                 Platform::putEnv('COMPOSER_DEV_MODE', $this->devMode ? '1' : '0');
             }