diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index 9fc5c318cf7e..dcff29097ba1 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -128,6 +128,8 @@ tcp_output(struct tcpcb *tp)
 	isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) != 0;
 #endif
 
+	mtx_assert(&tp->t_inpcb->inp_mtx, MA_OWNED);
+
 	/*
 	 * Determine length of data that should be transmitted,
 	 * and flags that will be used.