diff --git a/core/modules/migrate/src/Plugin/migrate/process/Substr.php b/core/modules/migrate/src/Plugin/migrate/process/Substr.php
index 8ebc98c..fe99e59 100644
--- a/core/modules/migrate/src/Plugin/migrate/process/Substr.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/Substr.php
@@ -23,6 +23,11 @@ class Substr extends ProcessPluginBase {
    * {@inheritdoc}
    */
   public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    // Don't fail if $value is NULL. For instance during a stub-migration.
+    if (is_null($value)) {
+      return $value;
+    }
+    
     $start = isset($this->configuration['start']) ? $this->configuration['start'] : 0;
     if (!is_int($start)) {
       throw new MigrateException('The start position configuration value should be an integer. Omit this key to capture from the beginning of the string.');