diff --git a/src/Extensions/ComposerLoaderExtension.php b/src/Extensions/ComposerLoaderExtension.php
index 700d4a5..363a71a 100644
--- a/src/Extensions/ComposerLoaderExtension.php
+++ b/src/Extensions/ComposerLoaderExtension.php
@@ -111,8 +111,12 @@ protected function getInstalledConstraint(BaseRepository $repository, $packageNa
     public function onAfterBuild()
     {
         // Mock COMPOSER_HOME if it's not defined already. Composer requires one of the two to be set.
-        if (!Environment::getEnv('HOME') && !Environment::getEnv('COMPOSER_HOME')) {
-            putenv('COMPOSER_HOME=/tmp');
+        if (!Environment::getEnv('COMPOSER_HOME')) {
+            $home = Environment::getEnv('HOME');
+            if (!$home || !is_dir($home) || !is_writable($home)) {
+                // Set our own directory
+                putenv('COMPOSER_HOME=' . sys_get_temp_dir());
+            }
         }
 
         $originalDir = getcwd();