--- kern_kse.c~	Tue Aug 10 03:16:17 2004
+++ kern_kse.c	Fri Aug 13 22:23:25 2004
@@ -995,7 +995,9 @@
 	kg = td->td_ksegrp;
 	if (kg->kg_upquantum && ticks >= kg->kg_nextupcall) {
 		mtx_lock_spin(&sched_lock);
-		td->td_upcall->ku_flags |= KUF_DOUPCALL;
+		/* fuword can block, check again */
+		if (td->td_upcall)
+			td->td_upcall->ku_flags |= KUF_DOUPCALL;
 		mtx_unlock_spin(&sched_lock);
 	}
 	return (0);