diff --git a/sys/kern/kern_switch.c b/sys/kern/kern_switch.c
index ee38e88d9fcd..a8ead79984d7 100644
--- a/sys/kern/kern_switch.c
+++ b/sys/kern/kern_switch.c
@@ -443,6 +443,8 @@ critical_exit(void)
 {
 	struct thread *td;
 
+	KASSERT(td->td_critnest > 0,
+	    ("critical_exit:  critnest < 0"));
 	td = curthread;
 	if (td->td_critnest == 1) {
 		td->td_critnest = 0;