diff --git a/sys/dev/usb/uvideo.c b/sys/dev/usb/uvideo.c
index 692bf03b36a0..b641c0008f41 100644
--- a/sys/dev/usb/uvideo.c
+++ b/sys/dev/usb/uvideo.c
@@ -811,6 +811,7 @@ uvideo_init_control(struct uvideo_softc *sc,
 		if (uvdesc->bDescriptorSubtype < UDESC_INPUT_TERMINAL ||
 		    uvdesc->bDescriptorSubtype > UDESC_EXTENSION_UNIT)
 			continue;
+		KASSERT(nunits < 255);
 		++nunits;
 	}