Fix for #2541

library/Zend/Form/Form.php

@@ -559,9 +559,10 @@ protected function prepareValidationGroup(FieldsetInterface $formOrFieldset, arr
 
                 $value = $values;
             } else {
-                if (isset($data[$key])) {
-                    $this->prepareValidationGroup($fieldset, $data[$key], $validationGroup[$key]);
+                if (!isset($data[$key])) {
+                    $data[$key] = array();
                 }
+                $this->prepareValidationGroup($fieldset, $data[$key], $validationGroup[$key]);
             }
         }
     }

tests/ZendTest/Form/FormTest.php

@@ -1025,6 +1025,34 @@ public function testRemoveCollectionFromValidationGroupWhenZeroCountAndNoData()
         $this->assertTrue($this->form->isValid());
     }
 
+    public function testFieldsetValidationGroupStillPreparedWhenEmptyData()
+    {
+        $emptyData = array();
+
+        $this->populateForm();
+        $this->form->get('foobar')->add(array(
+            'type' => 'Zend\Form\Element\Collection',
+            'name' => 'categories',
+            'options' => array(
+                'count' => 0,
+                'target_element' => array(
+                    'type' => 'ZendTest\Form\TestAsset\CategoryFieldset'
+                )
+            )
+        ));
+
+        $this->form->setValidationGroup(array(
+            'foobar' => array(
+                'categories' => array(
+                    'name'
+                )
+            )
+        ));
+
+        $this->form->setData($emptyData);
+        $this->assertFalse($this->form->isValid());
+    }
+
     public function testApplyObjectInputFilterToBaseFieldsetAndApplyValidationGroup()
     {
         $fieldset = new Fieldset('foobar');