diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index d56b09a604..1c1ed7a377 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -3567,8 +3567,11 @@ tramp-handle-substitute-in-file-name
     ;; First, we must replace environment variables.
     (setq filename (tramp-replace-environment-variables filename))
     (with-parsed-tramp-file-name filename nil
-      ;; We do not want to replace environment variables, again.
-      (let (process-environment)
+      ;; "//" has a special meaning at the beginning of a file name on
+      ;; Cygwin and MS-Windows.  And we do not want to replace
+      ;; environment variables, again.
+      (let ((system-type 'not-windows)
+            process-environment)
 	;; Ignore in LOCALNAME everything before "//" or "/~".
 	(when (stringp localname)
 	  (if (string-match "//\\(/\\|~\\)" localname)