diff --git a/core/modules/file/src/Plugin/migrate/process/d6/FileSource.php b/core/modules/file/src/Plugin/migrate/process/d6/FileSource.php
index 56bb07f..d277e4e 100644
--- a/core/modules/file/src/Plugin/migrate/process/d6/FileSource.php
+++ b/core/modules/file/src/Plugin/migrate/process/d6/FileSource.php
@@ -20,11 +20,13 @@ class FileSource extends ProcessPluginBase {
    */
   public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
     list($source_base_path, $filepath) = $value;
-    // If filepath is absolute, it overrides.
-    if (!empty($filepath) && $filepath[0] === '/') {
+
+    if (empty($source_base_path)) {
       return $filepath;
     }
-    return $source = $source_base_path . '/' . $filepath;
+
+    $source_base_path = rtrim($source_base_path, '/');
+    return $source_base_path . '/' . $filepath;
   }
 
 }
diff --git a/core/modules/file/tests/src/Kernel/Migrate/d6/FileMigrationTestTrait.php b/core/modules/file/tests/src/Kernel/Migrate/d6/FileMigrationTestTrait.php
index 14f77ed..28b1f10 100644
--- a/core/modules/file/tests/src/Kernel/Migrate/d6/FileMigrationTestTrait.php
+++ b/core/modules/file/tests/src/Kernel/Migrate/d6/FileMigrationTestTrait.php
@@ -28,7 +28,7 @@ protected function processMigration($migration) {
       // Make sure we have a single trailing slash.
       $source = $migration->getSourceConfiguration();
       $source['site_path'] = 'core/modules/simpletest';
-      $source['constants']['source_base_path'] = $this->root;
+      $source['constants']['source_base_path'] = '';
       $migration->set('source', $source);
     }
   }