diff --git a/drivers/usb/serial/mxuport.c b/drivers/usb/serial/mxuport.c
index ad5fdf55a02e..c9b9928c473a 100644
--- a/drivers/usb/serial/mxuport.c
+++ b/drivers/usb/serial/mxuport.c
@@ -962,6 +962,14 @@ static int mxuport_calc_num_ports(struct usb_serial *serial,
 	 */
 	BUILD_BUG_ON(ARRAY_SIZE(epds->bulk_out) < 16);
 
+	/*
+	 * The bulk-out buffers must be large enough for the four-byte header
+	 * (and following data), but assume anything smaller than eight bytes
+	 * is broken.
+	 */
+	if (usb_endpoint_maxp(epds->bulk_out[0]) < 8)
+		return -EINVAL;
+
 	for (i = 1; i < num_ports; ++i)
 		epds->bulk_out[i] = epds->bulk_out[0];