Controller Action ambiguity even with [HttpPost] attribute? (ASP.NET MVC4)

Question

So I have a "Register" view (that maps to a RegisterController), which is a simple form for people to register in my service. Given that the form's method is POST, then I can name both actions of the controller (the one that shows the form, and the one that receives its submission) the same name, but discriminate them via the [HttpPost] attribute, this way:

open System
open System.Web
open System.Web.Mvc
open System.Net
open System.Threading
open System.Configuration

[<HandleError>]
type RegisterController() =
    inherit Controller()

    member this.Index (guid: string) =
        let model = new RegisterModel(guid)
        this.View("Register", model) :> ActionResult

    [<HttpPost>]
    member this.Index
        (guid: string, password: string, passwordRetry: string) =
            if not (password.Equals(passwordRetry)) then
                raise(new Exception("Passwords must be the same"))
            WebDbAccess.SetNewPassword guid password
            RedirectResult("/") :> ActionResult

This works perfectly.

However, I just tried to use this same technique in the Home view of my app (for the Login functionality), and it doesn't work! It throws this:

 The current request for action 'Index' on controller type 'HomeController' is ambiguous between the following action methods:
System.Web.Mvc.ActionResult Index(System.String, System.String) on type FsWeb.Controllers.HomeController
System.Web.Mvc.ActionResult Index() on type FsWeb.Controllers.HomeController
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Reflection.AmbiguousMatchException: The current request for action 'Index' on controller type 'HomeController' is ambiguous between the following action methods:
System.Web.Mvc.ActionResult Index(System.String, System.String) on type FsWeb.Controllers.HomeController
System.Web.Mvc.ActionResult Index() on type FsWeb.Controllers.HomeController

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:


[AmbiguousMatchException: The current request for action 'Index' on controller type 'HomeController' is ambiguous between the following action methods:
System.Web.Mvc.ActionResult Index(System.String, System.String) on type FsWeb.Controllers.HomeController
System.Web.Mvc.ActionResult Index() on type FsWeb.Controllers.HomeController]
   System.Web.Mvc.Async.AsyncActionMethodSelector.FindAction(ControllerContext controllerContext, String actionName) +495852
   System.Web.Mvc.Async.ReflectedAsyncControllerDescriptor.FindAction(ControllerContext controllerContext, String actionName) +57
   System.Web.Mvc.ControllerActionInvoker.FindAction(ControllerContext controllerContext, ControllerDescriptor controllerDescriptor, String actionName) +16
   System.Web.Mvc.Async.AsyncControllerActionInvoker.BeginInvokeAction(ControllerContext controllerContext, String actionName, AsyncCallback callback, Object state) +114
   System.Web.Mvc.<>c__DisplayClass1d.<BeginExecuteCore>b__17(AsyncCallback asyncCallback, Object asyncState) +30
   System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +130
   System.Web.Mvc.Controller.BeginExecuteCore(AsyncCallback callback, Object state) +382
   System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +130
   System.Web.Mvc.Controller.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +317
   System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +15
   System.Web.Mvc.<>c__DisplayClass8.<BeginProcessRequest>b__2(AsyncCallback asyncCallback, Object asyncState) +71
   System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +130
   System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) +249
   System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) +50
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) +16
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +301
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155


Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.17929 

What's the difference between the 2 cases? I don't get it.

The HomeController's implementation is:

open System
open System.Web
open System.Web.Http

[<HandleError>]
type HomeController() =
    inherit Controller()

    [<HttpGet>]
    member this.Index () =
        this.View() :> ActionResult

    [<HttpPost>]
    member this.Index
        (id:string, password:string) =
            if (UserIsValid id password) then
                RedirectResult("lobby") :> ActionResult
            RedirectResult("loginError") :> ActionResult

Answer 1

@nemesv was right, I had to fully qualify HttpPost with System.Web.Mvc.HttpPost because otherwise it would use System.Web.Http.HttpPost!!