The check should be whether RPL_DIS_SEND is non-zero, not whether it is defined.

core/net/rpl/rpl-timers.c

@@ -70,7 +70,7 @@ handle_periodic_timer(void *ptr)
   rpl_recalculate_ranks();
 
   /* handle DIS */
-#ifdef RPL_DIS_SEND
+#if RPL_DIS_SEND
   next_dis++;
   if(rpl_get_any_dag() == NULL && next_dis >= RPL_DIS_INTERVAL) {
     next_dis = 0;