diff --git a/form/data_mappers.rst b/form/data_mappers.rst
index 15e66ce54b3..8bed0bd45c1 100644
--- a/form/data_mappers.rst
+++ b/form/data_mappers.rst
@@ -124,13 +124,27 @@ in your form type::
             /** @var FormInterface[] $forms */
             $forms = iterator_to_array($forms);
 
+            $colors = [
+                'red' => $forms['red']->getData(),
+                'green' => $forms['green']->getData(),
+                'blue' => $forms['blue']->getData(),
+            ];
+
+            // TransformationFailedException will be transformed into a FormError
+            foreach ($colors as $name => $color) {
+                if ($color > 256 || $color < 0) {
+                    throw new TransformationFailedException(sprintf('Invalid color %s "%s" value', $name, $color));
+                }
+            }
+
+
             // as data is passed by reference, overriding it will change it in
             // the form object as well
             // beware of type inconsistency, see caution below
             $viewData = new Color(
-                $forms['red']->getData(),
-                $forms['green']->getData(),
-                $forms['blue']->getData()
+                $colors['red'],
+                $colors['green'],
+                $colors['blue']
             );
         }
     }