Setting Resource.AvailableFrom with C# code raises COMException

Hello,

I am using:

Visual Studio 2010 10.0.30319.1

MS Project Professional 2010 14.0.6112.5000 (32-bit)

I am trying to set the AvailableFrom field of a Resource from a MSProject planning with the following piece of C# code:

using System;
using System.Collections.Generic;
using System.Text;
using System.Collections;
using MSProject = Microsoft.Office.Interop.MSProject;
using System.IO;
using System.Runtime.InteropServices;

namespace PlanningsSprint
{
    class MSProjectPlanningResources
    {
        public string InputFilename { get; set; }

        static int Main(string[] args)
        {
            MSProjectPlanningResources process = new MSProjectPlanningResources();
            process.InputFilename = "c:\\temp\\VS2010\\Project.mpp";
            process.Process();

            return 0;
        }


        public void Process()
        {

            try
            {
                MSProject.Application msProjectApplication = new MSProject.Application();
                msProjectApplication.FileOpenEx(InputFilename);

                MSProject.Resource resource = msProjectApplication.ActiveProject.Resources["ResourceName"];
                resource.AvailableFrom = new DateTime(2012, 1, 1);

                Console.ReadLine();
            }
            catch (Exception e)
            {
                Console.Error.WriteLine(e);
                Console.ReadLine();
            }
        }        
    }
}

It permanently fails with this exception:

System.Runtime.InteropServices.COMException (0x800A044D): The argument value is not valid.
   at Microsoft.Office.Interop.MSProject.Resource.set_AvailableFrom(Object retval)
   at PlanningsSprint.MSProjectPlanningResources.Process() in C:\Users\pdavid\Documents\SkyDrive\Visual Studio 2010\Projects\VS2010\MSProjectPlanningResources.cs:line 34

How can I fix this?

Patrick