diff --git a/src/Symfony/Component/Form/Extension/Core/DataTransformer/NumberToLocalizedStringTransformer.php b/src/Symfony/Component/Form/Extension/Core/DataTransformer/NumberToLocalizedStringTransformer.php
index 56f4c68bcf..0a290961b7 100644
--- a/src/Symfony/Component/Form/Extension/Core/DataTransformer/NumberToLocalizedStringTransformer.php
+++ b/src/Symfony/Component/Form/Extension/Core/DataTransformer/NumberToLocalizedStringTransformer.php
@@ -103,6 +103,10 @@ class NumberToLocalizedStringTransformer implements DataTransformerInterface
             return null;
         }
 
+        if ('NaN' === $value) {
+            throw new TransformationFailedException('"NaN" is not a valid number');
+        }
+
         $formatter = $this->getNumberFormatter();
         $value = $formatter->parse($value);
 
diff --git a/tests/Symfony/Tests/Component/Form/Extension/Core/DataTransformer/NumberToLocalizedStringTransformerTest.php b/tests/Symfony/Tests/Component/Form/Extension/Core/DataTransformer/NumberToLocalizedStringTransformerTest.php
index 91421765ec..d00aa599a1 100644
--- a/tests/Symfony/Tests/Component/Form/Extension/Core/DataTransformer/NumberToLocalizedStringTransformerTest.php
+++ b/tests/Symfony/Tests/Component/Form/Extension/Core/DataTransformer/NumberToLocalizedStringTransformerTest.php
@@ -128,10 +128,20 @@ class NumberToLocalizedStringTransformerTest extends LocalizedTestCase
      * @expectedException Symfony\Component\Form\Exception\TransformationFailedException
      * @link https://github.com/symfony/symfony/issues/3161
      */
-    public function testReverseTransformExpectsValidNumberFromNan()
+    public function testReverseTransformDisallowsNaN()
     {
         $transformer = new NumberToLocalizedStringTransformer();
 
         $transformer->reverseTransform('NaN');
     }
+
+    /**
+     * @expectedException Symfony\Component\Form\Exception\TransformationFailedException
+     */
+    public function testReverseTransformDisallowsNaN2()
+    {
+        $transformer = new NumberToLocalizedStringTransformer();
+
+        $transformer->reverseTransform('nan');
+    }
 }