diff --git a/module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php b/module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php
index de14969a036f5e5118b95d641c6010bf8314c779..98b40f6db3e91df1226cda42fef0841cc370e114 100644
--- a/module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php
+++ b/module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php
@@ -60,7 +60,7 @@ class Solr extends AbstractBase
      *
      * @var string
      */
-    protected $cacheDir;
+    protected $cacheDir = null;
 
     /**
      * Constructor.
@@ -75,7 +75,9 @@ class Solr extends AbstractBase
     ) {
         $this->solr = $solr;
         $this->driverFactory = $factory;
-        $this->cacheDir = rtrim($cacheDir, '/');
+        if (null !== $cacheDir) {
+            $this->cacheDir = rtrim($cacheDir, '/');
+        }
     }
 
     /**