diff --git a/src/Symfony/Component/Form/Extension/Core/ChoiceList/ObjectChoiceList.php b/src/Symfony/Component/Form/Extension/Core/ChoiceList/ObjectChoiceList.php
index 7c7aa0c768..0f1437db21 100644
--- a/src/Symfony/Component/Form/Extension/Core/ChoiceList/ObjectChoiceList.php
+++ b/src/Symfony/Component/Form/Extension/Core/ChoiceList/ObjectChoiceList.php
@@ -128,11 +128,13 @@ class ObjectChoiceList extends ChoiceList
                 if (null === $group) {
                     $groupedChoices[$i] = $choice;
                 } else {
-                    if (!isset($groupedChoices[$group])) {
-                        $groupedChoices[$group] = array();
+                    $groupName = (string) $group;
+
+                    if (!isset($groupedChoices[$groupName])) {
+                        $groupedChoices[$groupName] = array();
                     }
 
-                    $groupedChoices[$group][$i] = $choice;
+                    $groupedChoices[$groupName][$i] = $choice;
                 }
             }