diff --git a/packages/RDF/pretty_print.pl b/packages/RDF/pretty_print.pl
index 9b5f227c5..b7468ffca 100644
--- a/packages/RDF/pretty_print.pl
+++ b/packages/RDF/pretty_print.pl
@@ -14,6 +14,8 @@
 	  [ pretty_print/1
 	  ]).
 
+:- expects_dialect(swi).
+
 :- require([ atom_length/2
 	   , between/3
 	   , forall/2