Nhibernate default decimal (19,5) casting arithmetic overflow

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NHibernate

Hello,

I have the following piece of code

var result = session.Query<Employee>()
                    .GroupBy(x => x.Name)
                    .Select(s => new
                               {
                                sal = s.Sum(x =>x.Salary)
                               }
                           );

In My employee map i have my salary set to decimal(25,4).while querying the above, NH is generating the following

cast(sum(employee.Salary) as DECIMAL(19,5)) as col


As I have not given any particular map to the result , nhibernate is casting my result.sal to decimal(19,5) by default.However my data is containing more than 19 digits and this is resulting in an arithmetic overflow exception. Is there a way to avoid this,like asking NH to consider casting my result as (25,4) instead?