Index: backend/usb-libusb.c
===================================================================
--- backend/usb-libusb.c	(revision 9236)
+++ backend/usb-libusb.c	(working copy)
@@ -519,7 +519,8 @@
 
   if ((sern = cupsGetOption("SERIALNUMBER", num_values, values)) == NULL)
     if ((sern = cupsGetOption("SERN", num_values, values)) == NULL)
-      if ((sern = cupsGetOption("SN", num_values, values)) == NULL)
+      if ((sern = cupsGetOption("SN", num_values, values)) == NULL &&
+          printer->device->descriptor.iSerialNumber)
       {
        /*
         * Try getting the serial number from the device itself...