diff --git a/src/Symfony/Component/Debug/FatalErrorHandler/ClassNotFoundFatalErrorHandler.php b/src/Symfony/Component/Debug/FatalErrorHandler/ClassNotFoundFatalErrorHandler.php
index 1554a810a2..74b685cac1 100644
--- a/src/Symfony/Component/Debug/FatalErrorHandler/ClassNotFoundFatalErrorHandler.php
+++ b/src/Symfony/Component/Debug/FatalErrorHandler/ClassNotFoundFatalErrorHandler.php
@@ -108,9 +108,9 @@ class ClassNotFoundFatalErrorHandler implements FatalErrorHandlerInterface
             }
 
             if ($function[0] instanceof ComposerClassLoader || $function[0] instanceof SymfonyClassLoader) {
-                foreach ($function[0]->getPrefixes() as $paths) {
+                foreach ($function[0]->getPrefixes() as $prefix => $paths) {
                     foreach ($paths as $path) {
-                        $classes = array_merge($classes, $this->findClassInPath($path, $class));
+                        $classes = array_merge($classes, $this->findClassInPath($path, $class, $prefix));
                     }
                 }
             }
@@ -122,10 +122,11 @@ class ClassNotFoundFatalErrorHandler implements FatalErrorHandlerInterface
     /**
      * @param string $path
      * @param string $class
+     * @param string $prefix
      *
      * @return array
      */
-    private function findClassInPath($path, $class)
+    private function findClassInPath($path, $class, $prefix)
     {
         if (!$path = realpath($path)) {
             return array();
@@ -134,7 +135,7 @@ class ClassNotFoundFatalErrorHandler implements FatalErrorHandlerInterface
         $classes = array();
         $filename = $class.'.php';
         foreach (new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator($path), \RecursiveIteratorIterator::LEAVES_ONLY) as $file) {
-            if ($filename == $file->getFileName() && $class = $this->convertFileToClass($path, $file->getPathName())) {
+            if ($filename == $file->getFileName() && $class = $this->convertFileToClass($path, $file->getPathName(), $prefix)) {
                 $classes[] = $class;
             }
         }
@@ -145,27 +146,38 @@ class ClassNotFoundFatalErrorHandler implements FatalErrorHandlerInterface
     /**
      * @param string $path
      * @param string $file
+     * @param string $prefix
      *
      * @return string|null
      */
-    private function convertFileToClass($path, $file)
+    private function convertFileToClass($path, $file, $prefix)
     {
-        $namespacedClass = str_replace(array($path.DIRECTORY_SEPARATOR, '.php', '/'), array('', '', '\\'), $file);
-        $pearClass = str_replace('\\', '_', $namespacedClass);
+        $candidates = array(
+            // namespaced class
+            $namespacedClass = str_replace(array($path.DIRECTORY_SEPARATOR, '.php', '/'), array('', '', '\\'), $file),
+            // namespaced class (with target dir)
+            $namespacedClassTargetDir = $prefix.str_replace(array($path.DIRECTORY_SEPARATOR, '.php', '/'), array('', '', '\\'), $file),
+            // PEAR class
+            str_replace('\\', '_', $namespacedClass),
+            // PEAR class (with target dir)
+            str_replace('\\', '_', $namespacedClassTargetDir),
+        );
 
         // We cannot use the autoloader here as most of them use require; but if the class
         // is not found, the new autoloader call will require the file again leading to a
         // "cannot redeclare class" error.
-        if (!$this->classExists($namespacedClass) && !$this->classExists($pearClass)) {
-            require_once $file;
+        foreach ($candidates as $candidate) {
+            if ($this->classExists($candidate)) {
+                return $candidate;
+            }
         }
 
-        if ($this->classExists($namespacedClass)) {
-            return $namespacedClass;
-        }
+        require_once $file;
 
-        if ($this->classExists($pearClass)) {
-            return $pearClass;
+        foreach ($candidates as $candidate) {
+            if ($this->classExists($candidate)) {
+                return $candidate;
+            }
         }
     }