Index: file.c
===================================================================
--- file.c	(revision 42491)
+++ file.c	(working copy)
@@ -3848,7 +3848,7 @@
 	p = root;
     }
     if (p == name)
-	return rb_usascii_str_new2(".");
+	return rb_enc_str_new(".", strlen("."), enc);
 #ifdef DOSISH_DRIVE_LETTER
     if (has_drive_letter(name) && isdirsep(*(name + 2))) {
 	const char *top = skiproot(name + 2, end, enc);