New Post: Project Won't Build if Using SilentBootstrapperApplication

I have a single project componenet and the Wix# project to build the bootstrapper and installer in one solution. All works well if I simply accept the default bootstrapper application, but if I use the SilentBootstrapperApplication as show below, I get the errors also shown below when building the solution.

var msiFile = Compiler.BuildMsi(project);

var bootstrapper =
            new Bundle("MyProject",
                new MsiPackage(msiFile))
            {
                Version = assemblyVersion,
                UpgradeCode = new Guid("5ECB207C-4150-47F8-8A06-0E9509B7A5E4"),
                OutDir = Environment.GetEnvironmentVariable("OutDir") + @"\Installer\" + assemblyVersion,
                Manufacturer = "MyCo",
                Compressed = true
            };

        bootstrapper.Application = new SilentBootstrapperApplication();
        bootstrapper.Build();

<snip>The Windows Installer XML variable !(wix.WixMbaPrereqPackageId) is unknown</snip>
<snip>The Windows Installer XML variable !(wix.WixMbaPrereqLicenseUrl) is unknown</snip>

Where am I going wrong? I cannot see what I've missed. Other than that, I'm liking Wix# a lot!