--- sys/dev/usb/ugen.c	2008-11-22 14:40:13.000000000 +0800
+++ /var/current/mysrc/sys/dev/usb/ugen.c	2008-11-24 11:49:02.000000000 +0800
@@ -1396,6 +1396,8 @@
 		int error;
 
 		cdesc = ugen_get_cdesc(sc, fd->ufd_config_index, &len);
+		if (cdesc == NULL)
+			return (EINVAL);
 		if (len > fd->ufd_size)
 			len = fd->ufd_size;
 		iov.iov_base = (caddr_t)fd->ufd_data;