LINQ error - Invalid 'where' condition

Hi All.

I have console application for find/update date in production system.

I want to find records from Account with define condition:

(from account in targetDataContext.AccountSet

where account.gs1_PLN != null && account.gs1_PLN.Length > 0 && account.gs1_PLN.Trim().Length == 0
select new
{
Id = account.AccountId,
Pln_number = account.gs1_PLN
}));

But I get such error:

Invalid 'where' condition. An entity member is invoking an invalid property or method.

If I comment second and third part in "where" - it works.

How I can fixed this condition?

Thanks.