Index: head/sys/dev/random/fortuna.c
===================================================================
--- head/sys/dev/random/fortuna.c
+++ head/sys/dev/random/fortuna.c
@@ -310,6 +310,8 @@
 	u_int i;
 
 	RANDOM_RESEED_ASSERT_LOCK_OWNED();
+	KASSERT(!uint128_is_zero(fortuna_state.fs_counter), ("FS&K: C != 0"));
+
 	for (i = 0; i < blockcount; i++) {
 		/*-
 		 * FS&K - r = r|E(K,C)