diff --git a/src/Plugin/ExternalEntities/StorageClient/FileClientBase.php b/src/Plugin/ExternalEntities/StorageClient/FileClientBase.php
index e1a7ccf..af7985f 100644
--- a/src/Plugin/ExternalEntities/StorageClient/FileClientBase.php
+++ b/src/Plugin/ExternalEntities/StorageClient/FileClientBase.php
@@ -1046,6 +1046,7 @@ abstract class FileClientBase
    */
   public function generateFileFilterRegex(?string $structure = NULL) :string {
     $struct_re = $structure ?? $this->configuration['structure'];
+    $struct_re = is_null($struct_re) ? '' : $struct_re;
     // First, replace placeholders with user-provided regex.
     $pattern_re =
       '!\{' . static::SUBSTR_REGEXP . '(\w+)' . static::FIELDRE_REGEXP . '\}!'