diff --git a/lib/libefivar/efivar-dp-format.c b/lib/libefivar/efivar-dp-format.c
index 9c1969d81423..c3fa492d4c24 100644
--- a/lib/libefivar/efivar-dp-format.c
+++ b/lib/libefivar/efivar-dp-format.c
@@ -1033,7 +1033,7 @@ DevPathToTextUsbWWID (
 
   SerialNumberStr = (CHAR16 *) (&UsbWWId + 1);
   Length = (UINT16) ((DevicePathNodeLength ((EFI_DEVICE_PATH_PROTOCOL *) UsbWWId) - sizeof (USB_WWID_DEVICE_PATH)) / sizeof (CHAR16));
-  if (SerialNumberStr [Length - 1] != 0) {
+  if (Length >= 1 && SerialNumberStr [Length - 1] != 0) {
     //
     // In case no NULL terminator in SerialNumber, create a new one with NULL terminator
     //