bad operator

module/VuFind/src/VuFind/Cover/Loader.php

@@ -598,7 +598,7 @@ class Loader extends \VuFind\ImageLoader
             // Attempt to pull down the image:
             $result = $this->client->setUri($url)->send();
             if (!$result->isSuccess()) {
-                $this->debug("Failed to retrieve image from " + $url);
+                $this->debug('Failed to retrieve image from ' . $url);
                 return false;
             }
             $image = $result->getBody();