diff --git a/src/Rendering/Number.php b/src/Rendering/Number.php
index 6f9a0c8..dd41504 100644
--- a/src/Rendering/Number.php
+++ b/src/Rendering/Number.php
@@ -151,6 +151,9 @@ public function render($data, $citationNumber = null): string
      */
     public static function ordinal($num): string
     {
+        if (!is_numeric($num)) {
+            return $num;
+        }
         if ((int) ($num / 10) % 10 == 1) {
             $ordinalSuffix = CiteProc::getContext()->getLocale()->filter('terms', 'ordinal')->single;
         } elseif ($num % 10 == 1) {