diff --git a/C/tracer.c b/C/tracer.c
index 0ee421545..16048b976 100644
--- a/C/tracer.c
+++ b/C/tracer.c
@@ -54,7 +54,7 @@ send_tracer_message(char *start, char *name, Int arity, char *mname, CELL *args)
       if (args)
 	fprintf(Yap_stderr, "%s %s:%s(", start, mname, name);
       else
-	fprintf(Yap_stderr, "%s %s:%s/%d", start, mname, name, arity);
+	fprintf(Yap_stderr, "%s %s:%s/%ul", start, mname, name, (unsigned long int)arity);
     } else {
       fprintf(Yap_stderr, "%s %s:%s", start, mname, name);
     }