Consider zero a valid kernel version.

utility/vbutil_kernel.c

 /* Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  *
  * Verified boot kernel utility
  */
 
 #include <errno.h>
 #include <getopt.h>
 #include <inttypes.h>  /* For PRIu64 */
@@ skipping 732 matching lines @@
   return rv;
 }
 
 
 int main(int argc, char* argv[]) {
   char* filename = NULL;
   char* oldfile = NULL;
   char* key_block_file = NULL;
   char* signpubkey = NULL;
   char* signprivate = NULL;
-  uint64_t version = 0;
+  int version = -1;
   char* vmlinuz = NULL;
   char* bootloader = NULL;
   char* config_file = NULL;
   int vblockonly = 0;
   int verbose = 0;
   uint64_t pad = DEFAULT_PADDING;
   int mode = 0;
   int parse_error = 0;
   char* e;
   int i,r;
@@ skipping 91 matching lines @@
   switch(mode) {
     case OPT_MODE_PACK:
       bp = NewBlob(version, vmlinuz, bootloader, config_file);
       if (!bp)
         return 1;
       r = Pack(filename, key_block_file, signprivate, bp, pad, vblockonly);
       FreeBlob(bp);
       return r;
 
     case OPT_MODE_REPACK:
-      if (!config_file && !key_block_file && !version) {
+      if (!config_file && !key_block_file && (version<0)) {
         fprintf(stderr,
                 "You must supply at least one of "
                 "--config, --keyblock or --version\n");
         return 1;
       }
 
       bp = OldBlob(oldfile);
       if (!bp)
         return 1;
       r = ReplaceConfig(bp, config_file);
       if (!r) {
-        if (version) {
-          bp->kernel_version = version;
+        if (version >= 0) {
+                bp->kernel_version = (uint64_t) version;
         }
         r = Pack(filename, key_block_file, signprivate, bp, pad, vblockonly);
       }
       FreeBlob(bp);
       return r;
 
     case OPT_MODE_VERIFY:
       return Verify(filename, signpubkey, verbose);
 
     default:
       fprintf(stderr,
               "You must specify a mode: --pack, --repack or --verify\n");
       return PrintHelp(progname);
   }
 }