diff --git a/src/Symfony/Component/Config/Definition/ArrayNode.php b/src/Symfony/Component/Config/Definition/ArrayNode.php
index 451392257689..3b42ac0c2f0d 100644
--- a/src/Symfony/Component/Config/Definition/ArrayNode.php
+++ b/src/Symfony/Component/Config/Definition/ArrayNode.php
@@ -75,14 +75,17 @@ protected function preNormalize($value)
             return $value;
         }
 
+        $normalized = array();
+
         foreach ($value as $k => $v) {
             if (false !== strpos($k, '-') && false === strpos($k, '_') && !array_key_exists($normalizedKey = str_replace('-', '_', $k), $value)) {
-                $value[$normalizedKey] = $v;
-                unset($value[$k]);
+                $normalized[$normalizedKey] = $v;
+            } else {
+                $normalized[$k] = $v;
             }
         }
 
-        return $value;
+        return $normalized;
     }
 
     /**
diff --git a/src/Symfony/Component/Config/Tests/Definition/ArrayNodeTest.php b/src/Symfony/Component/Config/Tests/Definition/ArrayNodeTest.php
index 291c2fd2cce0..652a153a3d60 100644
--- a/src/Symfony/Component/Config/Tests/Definition/ArrayNodeTest.php
+++ b/src/Symfony/Component/Config/Tests/Definition/ArrayNodeTest.php
@@ -74,6 +74,10 @@ public function getPreNormalizationTests()
                 array('foo-bar_moo' => 'foo'),
                 array('foo-bar_moo' => 'foo'),
             ),
+            array(
+                array('anything-with-dash-and-no-underscore' => 'first', 'no_dash' => 'second'),
+                array('anything_with_dash_and_no_underscore' => 'first', 'no_dash' => 'second'),
+            ),
             array(
                 array('foo-bar' => null, 'foo_bar' => 'foo'),
                 array('foo-bar' => null, 'foo_bar' => 'foo'),