diff --git a/support/startup/preamble-utils.php b/support/startup/preamble-utils.php
--- a/support/startup/preamble-utils.php
+++ b/support/startup/preamble-utils.php
@@ -21,7 +21,7 @@
   }
 
   $forwarded_for = $_SERVER['HTTP_X_FORWARDED_FOR'];
-  if (!phutil_nonempty_string($forwarded_for)) {
+  if (!is_string($forwarded_for) || $forwarded_for === '') {
     return;
   }