Index: head/sys/netinet/tcp_subr.c
===================================================================
--- head/sys/netinet/tcp_subr.c
+++ head/sys/netinet/tcp_subr.c
@@ -798,8 +798,12 @@
 		}
 	}
 
+	if (blk->tfb_flags & TCP_FUNC_BEING_REMOVED) {
+		*num_names = 0;
+		return (EINVAL);
+	}
+
 	refcount_init(&blk->tfb_refcnt, 0);
-	blk->tfb_flags = 0;
 	blk->tfb_id = atomic_fetchadd_int(&next_tcp_stack_id, 1);
 	for (i = 0; i < *num_names; i++) {
 		n = malloc(sizeof(struct tcp_function), M_TCPFUNCTIONS, wait);