diff --git a/Form/ChoiceList/ModelChoiceList.php b/Form/ChoiceList/ModelChoiceList.php
index 140105d..fbd26a8 100644
--- a/Form/ChoiceList/ModelChoiceList.php
+++ b/Form/ChoiceList/ModelChoiceList.php
@@ -405,19 +405,14 @@ class ModelChoiceList extends ObjectChoiceList
      */
     private function getIdentifierValues($model)
     {
-        if ($model instanceof ActiveRecordInterface) {
-            return array($model->getPrimaryKey());
-        }
-
-        // readonly="true" models do not implement ActiveRecordInterface.
-        if ($model instanceof ActiveRecordInterface && method_exists($model, 'getPrimaryKey')) {
-            return array($model->getPrimaryKey());
-        }
-
         if (null === $model) {
             return array();
         }
 
+        if ($model instanceof ActiveRecordInterface) {
+            return array($model->getPrimaryKey());
+        }
+
         return $model->getPrimaryKeys();
     }