diff --git a/tools/gen_usb_descriptor.py b/tools/gen_usb_descriptor.py
index 36c6f45a6e..bd77be5832 100644
--- a/tools/gen_usb_descriptor.py
+++ b/tools/gen_usb_descriptor.py
@@ -757,11 +757,7 @@ const tusb_desc_webusb_url_t desc_webusb_url =
 
 // These next two hardcoded descriptor were pulled from the usb_descriptor.c file
 // of the tinyusb webusb_serial demo. TODO - this is probably something else to
-// integrate into the adafruit_usb_descriptors project, especially with this next
-// #define..
-
-// SWAG for now. Will have to be manually corrected until generated by gen_usb_descriptor.py
-#define ITF_NUM_VENDOR   3
+// integrate into the adafruit_usb_descriptors project...
 
 //--------------------------------------------------------------------+
 // BOS Descriptor
@@ -804,6 +800,8 @@ uint8_t const * tud_descriptor_bos_cb(void)
 }}
 
 
+#define ITF_NUM_VENOR   {webusb_interface} // used in this next descriptor
+
 uint8_t const desc_ms_os_20[] =
 {{
   // Set header: length, type, windows version, total length
@@ -836,4 +834,4 @@ TU_VERIFY_STATIC(sizeof(desc_ms_os_20) == MS_OS_20_DESC_LEN, "Incorrect size");
 
 // End of section about desc_ms_os_20
 
-""".format(webusb_url=args.webusb_url))
+""".format(webusb_url=args.webusb_url, webusb_interface=vendor_interface.bInterfaceNumber))