diff --git a/sys/sys/sched.h b/sys/sys/sched.h
--- a/sys/sys/sched.h
+++ b/sys/sys/sched.h
@@ -180,6 +180,7 @@
 sched_unpin(void)
 {
 	atomic_interrupt_fence();
+	MPASS(curthread->td_pinned > 0);
 	curthread->td_pinned--;
 }