Throw error in call_residue_vars/2

src/lib/atts.pl

@@ -3,6 +3,7 @@
                  term_attributed_variables/2]).
 
 :- use_module(library(dcgs)).
+:- use_module(library(error)).
 :- use_module(library(terms)).
 
 /* represent the list of attributes belonging to a variable,
@@ -113,6 +114,7 @@ user:goal_expansion(Term, M:get_atts(Var, Attr)) :-
 :- meta_predicate call_residue_vars(0, ?).
 
 call_residue_vars(Goal, Vars) :-
+    can_be(list, Vars),
     '$get_attr_var_queue_delim'(B),
     call(Goal),
     '$get_attr_var_queue_beyond'(B, Vars).