diff --git a/src/PieCrust/Util/UriBuilder.php b/src/PieCrust/Util/UriBuilder.php
index 9034e340..a442b00b 100644
--- a/src/PieCrust/Util/UriBuilder.php
+++ b/src/PieCrust/Util/UriBuilder.php
@@ -148,6 +148,10 @@ public static function slugify($value, $method)
         if ($reserved == null)
             $reserved = preg_quote(":/?#[]@!$&'()*+,;=\\ ", '/');
 
+        // Turn the value into a string, as it could actually be an
+        // instance of `PagePropertyData` or some other thing from the template.
+        $value = (string)$value;
+
         // Apply the main slugify mode.
         $mode = ($method & 0x000f);
         switch ($mode)