How to use union in Entity Framework with C#

I have two variables Income and Expense, I want to union this two variables and put the result into a sumSalary variable, but I always get an error (the error in sumSalary at Income). Please help me solve this problem.

public void PayrollReport()
{
    var income = employeeList.SelectMany(a => a.IncomesModel,(a,b) => new 
            {
                a.Salary,
                a.HolidayAllowance,
                EmployeeNik = a.NIK,
                EmployeeName = a.FirstName,

                b.Amount,
                b.ModelType,
                b.PeriodTime
            });

    var expense = employeeList.SelectMany(a => a.ExpensesModel, (a, b) => new
            {
                EmployeeNik = a.NIK,
                EmployeeName = a.FirstName,

                b.Amount,
                b.ModelType
            });

    var sumSalary = income.Union(expense);
}