Issue #3422

https://github.com/composer/composer/issues/3422

src/Composer/Autoload/ClassLoader.php

@@ -56,7 +56,10 @@ class ClassLoader
 
     public function getPrefixes()
     {
-        return call_user_func_array('array_merge', $this->prefixesPsr0);
+        if (!empty($this->prefixesPsr0)) {
+            return call_user_func_array('array_merge', $this->prefixesPsr0);
+        }
+        return array();
     }
 
     public function getPrefixesPsr4()

tests/Composer/Test/Autoload/ClassLoaderTest.php

@@ -55,4 +55,13 @@ class ClassLoaderTest extends \PHPUnit_Framework_TestCase
             array('ShinyVendor\\ShinyPackage\\SubNamespace\\Bar', true),
         );
     }
+
+    /**
+     * getPrefixes method should return empty array if ClassLoader does not have any psr-0 configuration
+     */
+    public function testGetPrefixesWithNoPSR0Configuration()
+    {
+        $loader = new ClassLoader();
+        $this->assertEmpty($loader->getPrefixes());
+    }
 }