Index: Classes/MVC/Controller/Argument.php
===================================================================
--- Classes/MVC/Controller/Argument.php	(revision 2355)
+++ Classes/MVC/Controller/Argument.php	(working copy)
@@ -363,7 +363,7 @@
 			$transformedValue = $this->propertyMapper->map(array_keys($value), $value, $this->dataType);
 		}
 
-		if (!($transformedValue instanceof $this->dataType)) {
+		if (!($transformedValue instanceof $this->dataType) && !($transformedValue === NULL && !$this->isRequired())) {
 			throw new Tx_Extbase_MVC_Exception_InvalidArgumentValue('The value must be of type "' . $this->dataType . '", but was of type "' . (is_object($transformedValue) ? get_class($transformedValue) : gettype($transformedValue)) . '".', 1251730701);
 		}
 		return $transformedValue;