Failing to create an array of structs containing an array of structs

Foo.sol

pragma solidity ^0.4.18;

contract Foo {
  struct Baz {
    uint bazNum;
  }

  struct Bar {
    Baz[] bazzes;
    uint barNum;
  }

  Bar[] bars;

  function createBar(uint barNum)
    public
    returns (uint)
  {
    uint barId = 0;//bars.length;
    bars[barId].barNum = barNum;
    return barId;
  }

  function getBarNum(uint barId)
    public
    view
    returns (uint)
  {
    return bars[barId].barNum;
  }
}

TestFoo.sol

pragma solidity ^0.4.18;

import "truffle/Assert.sol";
import "truffle/DeployedAddresses.sol";
import "../contracts/Foo.sol";

contract TestFoo {
  Foo foo = Foo(DeployedAddresses.Foo());

  function testFoo() public {
    uint barId = foo.createBar(10);
    Assert.equal(barId, 0, "First Bar Id");
  }
}