using repository method and got problem with update data while changing password. System.Data.Entity.Validation.DbEntityValidationException

Question

I've got problem in repository pattern while saving. this is what I wrote in unit of work:

public void Save()
        {
            db.SaveChanges();
        }'

db is for my UserList table

when I write user name and try to change the password, the error shows this: System.Data.Entity.Validation.DbEntityValidationException

here is my code for changing the password:

using (UnitOfWork db = new UnitOfWork())
        {
            var q = db.UList.Get();
            if(q != null)
            {
                UserList U = new UserList()
                {
                    Password = NewPassText.Text
                };
                db.UList.Update(U);
                db.Save();
                this.Hide();
                ProFrm ProFrm = new ProFrm(); ProFrm.Show();
            }
            else
            {

            }