Vows Asynchronous Export Error

gistfile1.js

var assert = require('assert');
var exec = require('child_process').exec;
var vows = require('vows');


var exportTests = function () {
  var batch = {
    'When the tests are exported asynchronously': {
      topic: function () { return 42; },
      'they actually run': function (val) { assert.equal(val, 42); }
    }
  };
  
  // export the tests
  vows.describe('readability').addBatch(batch).export(module);
}

exec('sleep 1', function (err, stdout, stderr) {
  if (err) throw(err);
  
  exportTests();
});

If run by node (i.e., node asyncExport.js), then the tests run normally. If run by vows, then the vows runner complains that it cannot find any tests.