Assembly not saving correctly

I have some very simple code to generate an assembly and invoke a method on a contained type. The method gets called and runs correctly, however when I view the generated assembly using Reflector, I don't see the type.

Below is the sample code:

namespace ConsoleApplication2
{
    class Proggy
    {
        public static void Main(string[] args)
        {
            var ab = AppDomain.CurrentDomain.DefineDynamicAssembly(
                new AssemblyName() { Name = "MyAssembly" },
                AssemblyBuilderAccess.RunAndSave);
            var module = ab.DefineDynamicModule(ab.GetName().Name);
            var typeBuilder = module.DefineType("MyType");
            var ctr = typeBuilder.DefineConstructor(MethodAttributes.Public, 
                CallingConventions.Standard, Type.EmptyTypes);
            var ilgc = ctr.GetILGenerator();
            ilgc.Emit(OpCodes.Ldarg_0);
            ilgc.Emit(OpCodes.Call, typeof(object).GetConstructor(Type.EmptyTypes));
            ilgc.Emit(OpCodes.Ret);
            var method = typeBuilder.DefineMethod("MyMethod", MethodAttributes.Public,
                typeof(int), new[] { typeof(string) });
            var ilg = method.GetILGenerator();
            ilg.Emit(OpCodes.Ldarg_1);
            ilg.EmitCall(OpCodes.Callvirt, typeof(string).GetProperty("Length").GetGetMethod(),
                null);
            ilg.Emit(OpCodes.Ret);
            var type = typeBuilder.CreateType();
            ab.Save("mytestasm.dll");
            var inst = Activator.CreateInstance(type);
            Console.WriteLine(type.InvokeMember("MyMethod", BindingFlags.InvokeMethod, null, inst,
                new[] { "MyTestString" }));
            Console.ReadLine();
        }
    }
}

and here is the corresponding disassembly from Reflector:

.assembly MyAssembly
{
    .ver 0:0:0:0
    .hash algorithm 0x00008004
}
.module RefEmit_OnDiskManifestModule
// MVID: {0B944140-58D9-430E-A867-DE0AD0A8701F}
// Target Runtime Version: v2.0.50727

... and ...

{
    .class private auto ansi <Module>
    {
    }
}

Can anyone help me with getting the assembly properly saved?

The trick is to use a "persistable module" version of DefineDynamicModule method on the AssemblyBuilder instance. That is, instead of:

var module = ab.DefineDynamicModule(ab.GetName().Name);

use something like:

var module = ab.DefineDynamicModule(ab.GetName().Name, ab.GetName().Name + ".mod");

Thereafter the corresponding module appears in the assembly after saving.