Happens with any mapped entity, where aNumber is greater than the number of results:

Session.Query<Entity>().Skip(aNumber).Count()

Or an example case for us in pagination:

var recordsOnThisPage = Session.Query<Entity>().Skip((pageNumber - 1) * recordsPerPage).Take(recordsPerPage).Count()

Will result in:

System.ArgumentOutOfRangeException : Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource)
at System.Collections.Generic.List`1.System.Collections.IList.get_Item(Int32 index)
at NHibernate.Linq.DefaultQueryProvider.ExecuteQuery(NhLinqExpression nhLinqExpression, IQuery query, NhLinqExpression nhQuery)
at NHibernate.Linq.DefaultQueryProvider.Execute(Expression expression)
at NHibernate.Linq.DefaultQueryProvider.Execute[TResult](Expression expression)
at System.Linq.Queryable.Count[TSource](IQueryable`1 source)