src/HOL/Nominal/nominal_fresh_fun.ML

    val bvs = map_index (Bound o fst) ps;
 (* select variables of the right class *)
    val vs = filter (fn t => is_of_fs_name (fastype_of1 (Ts, t)))
      (term_frees goal @ bvs);
 (* build the tuple *)
-   val s = Library.foldr1 (fn (v, s) =>
-     HOLogic.pair_const (fastype_of1 (Ts, v)) (fastype_of1 (Ts, s)) $ v $ s)
-       vs;
+   val s = (Library.foldr1 (fn (v, s) =>
+     HOLogic.pair_const (fastype_of1 (Ts, v)) (fastype_of1 (Ts, s)) $ v $ s) vs) handle _ => HOLogic.unit ;
    val fs_name_thm = get_dyn_thm thy ("fs_"^atom_basename^"1") atom_basename;
    val at_name_inst_thm = get_dyn_thm thy ("at_"^atom_basename^"_inst") atom_basename;
    val exists_fresh' = at_name_inst_thm RS at_exists_fresh';
 (* find the variable we want to instantiate *)
    val x = hd (term_vars (prop_of exists_fresh'));