diff --git a/UPGRADE-2.1.md b/UPGRADE-2.1.md
index d8314e023c..3e76e8ac76 100644
--- a/UPGRADE-2.1.md
+++ b/UPGRADE-2.1.md
@@ -480,6 +480,43 @@
     ));
     ```
 
+  * The "data_class" option now *must* be set if a form maps to an object. If
+    you leave it empty, the form will expect an array or a scalar value and
+    fail with a corresponding exception.
+
+    Likewise, if a form maps to an array, the option *must* be left empty now.
+
+  * The mapping of property paths to arrays has changed.
+
+    Previously, a property path "street" mapped to both a field `$street` of
+    a class (or its accessors `getStreet()` and `setStreet()`) and an index
+    `['street']` of an array or an object implementing `\ArrayAccess`.
+
+    Now, the property path "street" only maps to a class field (or accessors),
+    while the property path "[street]" only maps to indices.
+
+    If you defined property paths manually in the "property_path" option, you
+    should revise them and adjust them if necessary.
+
+    Before:
+
+    ```
+    $builder->add('name', 'text', array(
+        'property_path' => 'address.street',
+    ));
+    ```
+
+    After (if the address object is an array):
+
+    ```
+    $builder->add('name', 'text', array(
+        'property_path' => 'address[street]',
+    ));
+    ```
+
+    If address is an object in this case, the code given in "Before"
+    works without changes.
+
 ### Validator
 
   * The methods `setMessage()`, `getMessageTemplate()` and
diff --git a/src/Symfony/Component/Form/CHANGELOG.md b/src/Symfony/Component/Form/CHANGELOG.md
index bc4b136e41..b78db354a4 100644
--- a/src/Symfony/Component/Form/CHANGELOG.md
+++ b/src/Symfony/Component/Form/CHANGELOG.md
@@ -71,3 +71,5 @@ CHANGELOG
  * labels don't display field attributes anymore. Label attributes can be
    passed in the "label_attr" option/variable
  * added option "mapped" which should be used instead of setting "property_path" to false
+ * "data_class" now *must* be set if a form maps to an object and should be left empty otherwise
+ * improved error mapping on forms