diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 9caf8455af6..0273eea26b8 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -463,9 +463,9 @@ the buffer's value of `default-directory'."
   (let ((root (expand-file-name (file-name-as-directory (project-root project))))
         bufs)
     (dolist (buf (buffer-list))
-      (when (string-prefix-p root (expand-file-name
-                                   (buffer-local-value 'default-directory buf)))
-        (push buf bufs)))
+      (let ((dir (buffer-local-value 'default-directory buf)))
+        (when (and dir (string-prefix-p root (expand-file-name dir)))
+          (push buf bufs))))
     (nreverse bufs)))
 
 (defgroup project-vc nil