diff --git a/module/VuFind/src/VuFind/RecordDriver/SolrDefault.php b/module/VuFind/src/VuFind/RecordDriver/SolrDefault.php
index 4b16b0c16dd9df5354880c8d9b901a0680abfadd..c910f4f42670be26eb7db966d0ed087f0fe032d4 100644
--- a/module/VuFind/src/VuFind/RecordDriver/SolrDefault.php
+++ b/module/VuFind/src/VuFind/RecordDriver/SolrDefault.php
@@ -800,7 +800,7 @@ class SolrDefault extends AbstractBase
      *
      * @return array
      */
-    protected function getUnknownFormatOpenUrlParams($format)
+    protected function getUnknownFormatOpenUrlParams($format = 'UnknownFormat')
     {
         $params = $this->getDefaultOpenUrlParams();
         $params['rft_val_fmt'] = 'info:ofi/fmt:kev:mtx:dc';