diff -u b/core/modules/migrate/tests/src/Unit/process/SubstrTest.php b/core/modules/migrate/tests/src/Unit/process/SubstrTest.php
--- b/core/modules/migrate/tests/src/Unit/process/SubstrTest.php
+++ b/core/modules/migrate/tests/src/Unit/process/SubstrTest.php
@@ -48,7 +48,7 @@
       // Tests without start value and valid length value.
       [NULL, 7, 'Captain'],
       // Tests with valid start value and no length value.
-      [1, NULL, 'Captain Janeway'],
+      [0, NULL, 'Captain Janeway'],
       // Tests without both start and length values.
       [NULL, NULL, 'Captain Janeway'],
     ];