diff --git a/.github/workflows/composer.yml b/.github/workflows/composer.yml
index 59d69b576..1831a7cf0 100644
--- a/.github/workflows/composer.yml
+++ b/.github/workflows/composer.yml
@@ -19,6 +19,7 @@ jobs:
       uses: shivammathur/setup-php@master
       with:
         php-version: 7.4
+        tools: composer:v1
         coverage: none
     - name: Update composer
       run: sudo composer self-update && composer --version
diff --git a/composer/ClassLoader.php b/composer/ClassLoader.php
index fce8549f0..03b9bb9c4 100644
--- a/composer/ClassLoader.php
+++ b/composer/ClassLoader.php
@@ -60,7 +60,7 @@ class ClassLoader
     public function getPrefixes()
     {
         if (!empty($this->prefixesPsr0)) {
-            return call_user_func_array('array_merge', $this->prefixesPsr0);
+            return call_user_func_array('array_merge', array_values($this->prefixesPsr0));
         }
 
         return array();