diff --git a/drivers/net/wireless/ath/ath6kl/htc_mbox.c b/drivers/net/wireless/ath/ath6kl/htc_mbox.c
index cd0e1ba410d6ac3050fa5bb3fd35f99451492238..ceaf92192658f1625acc114b05bc926ee5e56e41 100644
--- a/drivers/net/wireless/ath/ath6kl/htc_mbox.c
+++ b/drivers/net/wireless/ath/ath6kl/htc_mbox.c
@@ -2492,7 +2492,8 @@ static int ath6kl_htc_mbox_conn_service(struct htc_target *target,
 		max_msg_sz = le16_to_cpu(resp_msg->max_msg_sz);
 	}
 
-	if (assigned_ep >= ENDPOINT_MAX || !max_msg_sz) {
+	if (WARN_ON_ONCE(assigned_ep == ENDPOINT_UNUSED ||
+			 assigned_ep >= ENDPOINT_MAX || !max_msg_sz)) {
 		status = -ENOMEM;
 		goto fail_tx;
 	}