diff --git a/DependencyInjection/SonataFormatterExtension.php b/DependencyInjection/SonataFormatterExtension.php
index 1e4cb44a..3f83e70e 100644
--- a/DependencyInjection/SonataFormatterExtension.php
+++ b/DependencyInjection/SonataFormatterExtension.php
@@ -58,6 +58,11 @@ public function load(array $configs, ContainerBuilder $container)
 
         // NEXT_MAJOR: remove this if block
         if (!isset($config['default_formatter'])) {
+            @trigger_error(<<<'EOT'
+Not setting the default_formatter configuration node is deprecated since 3.x,
+and will no longer be supported in 4.0.
+EOT
+            , E_USER_DEPRECATED);
             reset($config['formatters']);
             $config['default_formatter'] = key($config['formatters']);
         }
diff --git a/UPGRADE-3.x.md b/UPGRADE-3.x.md
index fac2634f..dc563d84 100644
--- a/UPGRADE-3.x.md
+++ b/UPGRADE-3.x.md
@@ -1,6 +1,15 @@
 UPGRADE 3.x
 ===========
 
+## Deprecated unspecified default_formatter configuration node
+
+The `default_formatter` configuration node will become required.
+
+```yaml
+sonata_formatter:
+    default_formatter: my_formatter
+```
+
 UPGRADE FROM 3.0 to 3.1
 =======================