diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
--- a/sys/kern/uipc_socket.c
+++ b/sys/kern/uipc_socket.c
@@ -3841,12 +3841,23 @@
 				error = EINVAL;
 				goto bad;
 			}
+			SOCK_LOCK(so);
+			if (SOLISTENING(so)) {
+				/*
+				 * A socket's FIB must be set before calling
+				 * listen().
+				 */
+				error = EINVAL;
+				SOCK_UNLOCK(so);
+				goto bad;
+			}
 			if (((so->so_proto->pr_domain->dom_family == PF_INET) ||
 			   (so->so_proto->pr_domain->dom_family == PF_INET6) ||
 			   (so->so_proto->pr_domain->dom_family == PF_ROUTE)))
 				so->so_fibnum = optval;
 			else
 				so->so_fibnum = 0;
+			SOCK_UNLOCK(so);
 			break;
 
 		case SO_USER_COOKIE: