diff --git a/core/modules/user/src/Plugin/migrate/source/d6/User.php b/core/modules/user/src/Plugin/migrate/source/d6/User.php
index 99cdcdebdf425bac2b94e5c1b86bf4e446c4ba7c..b4e5b9b73ac5ed1ee773bae01ce99668177ea0b3 100644
--- a/core/modules/user/src/Plugin/migrate/source/d6/User.php
+++ b/core/modules/user/src/Plugin/migrate/source/d6/User.php
@@ -68,7 +68,10 @@ public function prepareRow(Row $row) {
     }
 
     // Unserialize Data.
-    $row->setSourceProperty('data', unserialize($row->getSourceProperty('data')));
+    $data = $row->getSourceProperty('data');
+    if ($data !== NULL) {
+      $row->setSourceProperty('data', unserialize($row->getSourceProperty('data')));
+    }
 
     return parent::prepareRow($row);
   }