收到错误：在需要根据以下语法提供条件的上下文中指定的非布尔类型的表达式。

string Query = "SELECT User.First, User.Last, 
                   Details.Address1, Details.Address2, 
                   Details.County, Details.Country 
                FROM User 
                INNER JOIN Details 
                   on User.Details_DetailsID 
                WHERE User.Last ='" + cbUsers.SelectedItem.ToString() + "'";