DataAnnotations.Validator not validating property

As part of a larger project I wrote this sample code:

using Microsoft.VisualStudio.TestPlatform.UnitTestFramework;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace UnitTestLibrary
{
    public class temp
    {
        private System.Int32 _someNumber;

        [Range(1,5)]
        public System.Int32 somenumber
        {
            get
            {
                return _someNumber;
            }
            set
            {
                if(_someNumber != value)
                {
                    _someNumber = value;
                }
            }
        }

        public bool ValidateProperty()
        {
            //do validation of a single property
            List<ValidationResult> validationResults = new List<ValidationResult>();
           
            //works as expected
            bool result = Validator.TryValidateObject(this, new ValidationContext(this), validationResults);

            //throws System.ArgumentException: The value for property 'somenumber' must be of type System.Int32
            ValidationContext validationContext = new ValidationContext(this, null)
            {
                MemberName = "somenumber"
            };

            result = Validator.TryValidateProperty(this, validationContext, validationResults);

            return result;
        }
    }

    [TestClass]
    public class temptest
    {
        [TestMethod]
        public void testmethod()
        {
            temp tempa = new temp();
            tempa.somenumber = (System.Int32)3;
            Assert.IsTrue(tempa.ValidateProperty());
        }
    }
}

why do I get an exception on the type of the property?

Maybe important: I use this in the context of Windows RT unit test project using Visual Studio 2013 express for Windows (store apps).

thanks