Invalid Program Exception with "Enable 32-bit Applications" set to false

Invalid Program Exception with "Enable 32-bit Applications" set to false

I am generating my own proxy that wraps objects returned from MongoDB. The
proxy implements an interface:
interface IProxy
{
string __ID {get;}
}
The Proxy Generator uses the following code to generate the implementation
PropertyBuilder proxyID = typeBuilder.DefineProperty("__ID",
PropertyAttributes.None, typeof(string), null);
proxyID.SetCustomAttribute(new
CustomAttributeBuilder(typeof(Root.Attributes.DataAnnotations.NotMappedAttribute).GetConstructor(Type.EmptyTypes),
new object[0]));
MethodBuilder proxyID_PropertyGet = typeBuilder.DefineMethod("get___ID",
MethodAttributes.Virtual | MethodAttributes.Public |
MethodAttributes.SpecialName | MethodAttributes.HideBySig,
typeof(string), Type.EmptyTypes);
ILGenerator __IDILget = proxyID_PropertyGet.GetILGenerator();
__IDILget.Emit(OpCodes.Ldarg_0);
__IDILget.Emit(OpCodes.Ldfld, objectID);
__IDILget.Emit(OpCodes.Callvirt, typeof(ObjectId).GetMethod("ToString",
BindingFlags.Public | BindingFlags.Instance));
__IDILget.Emit(OpCodes.Ret);
proxyID.SetGetMethod(proxyID_PropertyGet);
The application is compiled with the "Any CPU" configuration.
When running on our development machines using the Visual Studio web
server, the application works fine.
When running on IIS7.5 (Windows 2008 R2), it throws an invalid program
exception whenever the __ID property is accessed. Changing the "Enable
32-bit applications" setting to true changes this behavior.
I'd like to not have to modify the IIS configuration. Why does the
exception only get thrown in 64-bit applications?