diff --git a/src/Controller/ImageStyleDownloadController.php b/src/Controller/ImageStyleDownloadController.php
index c8c9d673f829450430048eab607a22163e93f6e6..704ceea56ea5571a40b45d3c4f8e4d1f22d6e3de 100644
--- a/src/Controller/ImageStyleDownloadController.php
+++ b/src/Controller/ImageStyleDownloadController.php
@@ -40,7 +40,7 @@ class ImageStyleDownloadController extends CoreImageStyleDownloadController {
     }
     $path_info = pathinfo($target);
     // If .webp file, look for image to derive from.
-    if ($path_info['extension'] == 'webp') {
+    if (isset($path_info['extension']) && $path_info['extension'] === 'webp') {
       $image_uri = $scheme . '://' . $target;
       // Continue processing if source found, else throw NotFoundHttpException.
       if ($source_uri = $this->lookupSourceImage($image_uri)) {