diff --git a/src/Symfony/Component/Form/Tests/Extension/Core/DataTransformer/DateTimeToStringTransformerTest.php b/src/Symfony/Component/Form/Tests/Extension/Core/DataTransformer/DateTimeToStringTransformerTest.php
index 449219ffd3..aabad81f56 100644
--- a/src/Symfony/Component/Form/Tests/Extension/Core/DataTransformer/DateTimeToStringTransformerTest.php
+++ b/src/Symfony/Component/Form/Tests/Extension/Core/DataTransformer/DateTimeToStringTransformerTest.php
@@ -23,7 +23,7 @@ class DateTimeToStringTransformerTest extends DateTimeTestCase
             array('Y-m-d H:i', '2010-02-03 16:05', '2010-02-03 16:05:00 UTC'),
             array('Y-m-d H', '2010-02-03 16', '2010-02-03 16:00:00 UTC'),
             array('Y-m-d', '2010-02-03', '2010-02-03 00:00:00 UTC'),
-            array('Y-m', '2010-02', '2010-02-01 00:00:00 UTC'),
+            array('Y-m', '2010-12', '2010-12-01 00:00:00 UTC'),
             array('Y', '2010', '2010-01-01 00:00:00 UTC'),
             array('d-m-Y', '03-02-2010', '2010-02-03 00:00:00 UTC'),
             array('H:i:s', '16:05:06', '1970-01-01 16:05:06 UTC'),