diff --git a/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c b/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
index 3c6ce6f3d93e..1f8ed7ff31e5 100644
--- a/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
+++ b/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
@@ -847,7 +847,9 @@ static irqreturn_t extron_interrupt(struct serio *serio, unsigned char data,
 			return IRQ_HANDLED;
 		memcpy(extron->data, extron->buf, extron->idx);
 		extron->len = extron->idx;
-		extron->data[extron->len] = 0;
+		/* Keep fixed-offset response tests from using stale tail bytes. */
+		memset(extron->data + extron->len, 0,
+		       min_t(size_t, 10, sizeof(extron->data) - extron->len));
 		if (debug)
 			dev_info(extron->dev, "received %s\n", extron->data);
 		extron->idx = 0;