diff --git a/collects/scribble/html-render.rkt b/collects/scribble/html-render.rkt
index cf33a004..e8f8ad4d 100644
--- a/collects/scribble/html-render.rkt
+++ b/collects/scribble/html-render.rkt
@@ -1597,7 +1597,7 @@
                    (list name)))))))
 
 (define in-plt?
-  (let ([roots (map explode (list (find-doc-dir) (find-collects-dir)))])
+  (let ([roots (map explode (filter values (list (find-doc-dir) (find-collects-dir))))])
     (lambda (path)
       (ormap (lambda (root)
                (let loop ([path  path] [root root])