diff --git a/sys/i386/i386/pmap.c b/sys/i386/i386/pmap.c
index d0699b5311a9..a09138cb2567 100644
--- a/sys/i386/i386/pmap.c
+++ b/sys/i386/i386/pmap.c
@@ -1947,6 +1947,7 @@ pmap_lazyfix(pmap_t pmap)
 
 		/* Find least significant set bit. */
 		lsb = cpusetobj_ffs(&mask);
+		MPASS(lsb != 0);
 		lsb--;
 		CPU_SETOF(lsb, &mask);
 		mtx_lock_spin(&smp_ipi_mtx);
diff --git a/sys/i386/xen/pmap.c b/sys/i386/xen/pmap.c
index 1149b6f516c4..2f3953884ab1 100644
--- a/sys/i386/xen/pmap.c
+++ b/sys/i386/xen/pmap.c
@@ -1730,6 +1730,7 @@ pmap_lazyfix(pmap_t pmap)
 
 		/* Find least significant set bit. */
 		lsb = cpusetobj_ffs(&mask);
+		MPASS(lsb != 0);
 		lsb--;
 		CPU_SETOF(lsb, &mask);
 		mtx_lock_spin(&smp_ipi_mtx);