diff --git a/src/lib/format.pl b/src/lib/format.pl
index a3a0f5a4..811a4c33 100644
--- a/src/lib/format.pl
+++ b/src/lib/format.pl
@@ -460,7 +460,7 @@ portray_clause(Term) :-
 
 portray_clause(Stream, Term) :-
         phrase(portray_clause_(Term), Ls),
-        maplist(put_char(Stream), Ls).
+        format(Stream, "~s", [Ls]).
 
 portray_clause_(Term) -->
         { term_variables(Term, Vs),