--- a/lisp/files.el
+++ b/lisp/files.el
@@ -3567,8 +3569,10 @@
 (defun dir-locals-collect-variables (class-variables root variables)
   "Collect entries from CLASS-VARIABLES into VARIABLES.
 ROOT is the root directory of the project.
 Return the new variables list."
-  (let* ((file-name (buffer-file-name))
+  (let* ((file-name (or (buffer-file-name)
+			;; handle non-file buffers, too
+			default-directory))
 	 (sub-file-name (if file-name
                             ;; FIXME: Why not use file-relative-name?
 			    (substring file-name (length root)))))