diff --git a/sys/usb/usbus/usbus_fmt.c b/sys/usb/usbus/usbus_fmt.c
index 2e54fdbe41..069c09cc46 100644
--- a/sys/usb/usbus/usbus_fmt.c
+++ b/sys/usb/usbus/usbus_fmt.c
@@ -199,6 +199,7 @@ static size_t _fmt_descriptors_iface_alts(usbus_t *usbus,
          alt = alt->next) {
         usb_descriptor_interface_t usb_iface;
         _fmt_descriptor_iface(iface, &usb_iface);
+        len += sizeof(usb_descriptor_interface_t);
         usb_iface.alternate_setting = alts++;
         usb_iface.num_endpoints = _num_endpoints_alt(alt);
         usbus_control_slicer_put_bytes(usbus, (uint8_t *)&usb_iface,