How can I mock some variables in the private static method?

 public class Foo
{
    private static int Parse(int a, string c)
    {
        return 3;
    }
}

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
       //ARRANGE - faking private static method
       Isolate.NonPublic.WhenCalled(typeof (Foo) , "Parse").WillReturn(5);

       //ACT - invoking private static method
       var num =  Isolate.Invoke.Method(typeof (Foo), "Parse", new object[] {1, "something"});

       //use one of the ASSERTs below
       Assert.AreEqual(5 , num);

       //verifying private static method
       Isolate.Verify.NonPublic.WasCalled(typeof(Foo) , "Parse");
    }
}