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.');
diff --git a/core/modules/migrate/tests/src/Unit/process/SubstrTest.php b/core/modules/migrate/tests/src/Unit/process/SubstrTest.php
index fb84509..439e5aa 100644
--- a/core/modules/migrate/tests/src/Unit/process/SubstrTest.php
+++ b/core/modules/migrate/tests/src/Unit/process/SubstrTest.php
@@ -89,4 +89,13 @@ public function testLengthIsString() {
     $this->plugin->transform(['foo'], $this->migrateExecutable, $this->row, 'destinationproperty');
   }
 
+  /**
+   * Tests that Substr does not throw an Exception when the value is NULL
+   */
+  public function testSubstrNull() {
+    $configuration = [];
+    $this->plugin = new Substr($configuration, 'map', []);
+    $value = $this->plugin->transform(NULL, $this->migrateExecutable, $this->row, 'destinationproperty');
+    $this->assertNull($value);
+  }
 }