diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 4e4a71307d63c86c2f330492540a04c96ebd32c9..6fbc54957fb44a255848882bbe8e261735214a8f 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -360,7 +360,7 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf)
 static void acm_ctrl_irq(struct urb *urb)
 {
 	struct acm *acm = urb->context;
-	struct usb_cdc_notification *dr = urb->transfer_buffer;
+	struct usb_cdc_notification *dr;
 	unsigned int current_size = urb->actual_length;
 	unsigned int expected_size, copy_size, alloc_size;
 	int retval;
@@ -387,9 +387,20 @@ static void acm_ctrl_irq(struct urb *urb)
 
 	usb_mark_last_busy(acm->dev);
 
-	if (acm->nb_index)
+	if (acm->nb_index == 0) {
+		/*
+		 * The first chunk of a message must contain at least the
+		 * notification header with the length field, otherwise we
+		 * can't get an expected_size.
+		 */
+		if (current_size < sizeof(struct usb_cdc_notification)) {
+			dev_dbg(&acm->control->dev, "urb too short\n");
+			goto exit;
+		}
+		dr = urb->transfer_buffer;
+	} else {
 		dr = (struct usb_cdc_notification *)acm->notification_buffer;
-
+	}
 	/* size = notification-header + (optional) data */
 	expected_size = sizeof(struct usb_cdc_notification) +
 					le16_to_cpu(dr->wLength);