NHibernate possible bug in IQuery.List<T>()

recently, we had to clean the database from all the testing data, when an error similar to the following appeared:

The value "" is not of type “System.Nullable`1[System.DateTime]” and cannot be used in this generic collection

The code I executed was:

IQuery query = DbManager.MySession.CreateQuery("select max(dateObject.EndDate) from DateDomain dateObject");
IList<DateTime?> list = query.List<DateTime?>();

When I debugged NHibernate code, I reached to the following AddAll() method in the ArrayHelper class:

// NH-specific
public static void AddAll(IList to, IList from)
{
    foreach (object obj in from)
    {
        to.Add(obj);
    }
}

You can find the explanation of the error here.
Which brings us to the interesting question: why the implementation of IList Add method doesn’t consider the “nullability” of the T object? and why the parameter is of type IList?!

Am I missing something? should I report it as a bug?