diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -1955,7 +1955,7 @@
 				tp->t_flags |= TF_DELACK;
 			} else {
 				tp->t_flags |= TF_ACKNOW;
-				tcp_output(tp);
+				(void) tcp_output(tp);
 			}
 			goto check_delack;
 		}
diff --git a/sys/netinet/tcp_timewait.c b/sys/netinet/tcp_timewait.c
--- a/sys/netinet/tcp_timewait.c
+++ b/sys/netinet/tcp_timewait.c
@@ -123,7 +123,7 @@
 	soisdisconnected(inp->inp_socket);
 
 	if (tp->t_flags & TF_ACKNOW)
-		tcp_output(tp);
+		(void) tcp_output(tp);
 
 	if (V_nolocaltimewait && (
 #ifdef INET6