diff --git a/gump.class.php b/gump.class.php
index a8115f0..724d905 100644
--- a/gump.class.php
+++ b/gump.class.php
@@ -1126,7 +1126,7 @@ protected function filter_sanitize_string($value, array $params = [])
      */
     private static function polyfill_filter_var_string($value)
     {
-        $str = preg_replace('/\x00|<[^>]*>?/', '', $value);
+        $str = preg_replace('/\x00|<[^>]*>?/', '', is_null($value) ? '' : $value);
 
         return (string)str_replace(["'", '"'], ['&#39;', '&#34;'], $str);
     }