diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 7c1a92f..4268cd1 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1160,7 +1160,8 @@
 	int first_sack_index;
 
 	if (!tp->sacked_out) {
-		tp->fackets_out = 0;
+		if (WARN_ON(tp->fackets_out))
+			tp->fackets_out = 0;
 		tp->highest_sack = tp->snd_una;
 	}
 	prior_fackets = tp->fackets_out;