diff --git a/src/lib/clpz.pl b/src/lib/clpz.pl
index 8f9c6bf8..9b162a19 100644
--- a/src/lib/clpz.pl
+++ b/src/lib/clpz.pl
@@ -6168,8 +6168,7 @@ with_local_attributes(Vars, Goal, Result) :-
                % we made during propagation, and unify the variables
                % in the thrown copy with Vars in order to get the
                % intended variables in Result.
-               asserta(nat_copy(Vars-Result)),
-               retract(nat_copy(Copy)),
+               copy_term_nat(Vars-Result, Copy),
                throw(local_attributes(Copy))),
               local_attributes(Vars-Result),
               true).