diff --git a/gdk/broadway/gdkbroadway-server.c b/gdk/broadway/gdkbroadway-server.c
index cb9bdf4664..adc28a0b72 100644
--- a/gdk/broadway/gdkbroadway-server.c
+++ b/gdk/broadway/gdkbroadway-server.c
@@ -506,7 +506,7 @@ make_valid_fs_char (char c)
 {
   char chars[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890";
 
-  return chars[c % sizeof (chars)];
+  return chars[c % (sizeof (chars) - 1)];
 }
 
 /* name must have at least space for 34 bytes */