Passes in C# fails in Mono - looks like different behaviour in Linq rather than Iterator blocks

EnumerableExtensions.cs

public static IEnumerable<T> HandleExceptions<T>(this IEnumerable<T> values, Func<Exception, T> onException)
{
    using (var enumerator = values.GetEnumerator())
    {
        bool next = true;
        Exception lastException = null;
        while (next)
        {
            try
            {
                next = enumerator.MoveNext();
            }
            catch (Exception ex)
            {
                lastException = ex;
            }

            if (lastException != null)
            {
                yield return onException(lastException);

                lastException = null;
            }
            else if (next)
            {
                yield return enumerator.Current;
            }
        }
    }
}

EnumerableExtensionsTests.cs

[Test]
public void HandleExceptions()
{
    IList<Exception> exceptions = new List<Exception>();
    Func<Exception, int?> callback = (ex) => { exceptions.Add(ex); return (int?)0; };
    var result = Enumerable.Range(0, 5).Select(x => { 
            if (x%2 == 0) 
                throw new Exception("Exception" + x);
            return (int?)x;
        }).HandleExceptions(callback).ToList();
    Assert.AreEqual(5,result.Count);
    Assert.AreEqual(3,exceptions.Count);
}