diff --git a/libraries/src/Form/Rule/SubFormRule.php b/libraries/src/Form/Rule/SubFormRule.php
index a3f9cb07d7b36..9436c07069908 100644
--- a/libraries/src/Form/Rule/SubFormRule.php
+++ b/libraries/src/Form/Rule/SubFormRule.php
@@ -45,7 +45,12 @@ public function test(\SimpleXMLElement $element, $value, $group = null, Registry
 		{
 			throw new \UnexpectedValueException(sprintf('%s is no subform field.', $element['name']));
 		}
-
+		
+		if ($value === null)
+		{
+			return true;
+		}
+		
 		$subForm = $field->loadSubForm();
 
 		// Multiple values: Validate every row.