diff --git a/sys/kern/subr_trap.c b/sys/kern/subr_trap.c
--- a/sys/kern/subr_trap.c
+++ b/sys/kern/subr_trap.c
@@ -256,6 +256,7 @@
 {
 	THREAD_LOCK_ASSERT(td, MA_OWNED);
 	MPASS(tda < TDA_MAX);
+	MPASS(__bitcount(tda) == 1);
 
 	td->td_ast |= TDAI(tda);
 }
@@ -265,6 +266,7 @@
 {
 	THREAD_LOCK_ASSERT(td, MA_OWNED);
 	MPASS(tda < TDA_MAX);
+	MPASS(__bitcount(tda) == 1);
 
 	td->td_ast &= ~TDAI(tda);
 }