diff --git a/desc/usb_config.json b/desc/usb_config.json
index 2630cf6..a08fa28 100644
--- a/desc/usb_config.json
+++ b/desc/usb_config.json
@@ -29,7 +29,9 @@
               "type": "acmfd"
             },
             {
-              "type": "ufd"
+              "type": "ufd",
+              "master_interface": 0,
+              "slave_interface": 1
             },
             {
               "type": "cmfd",