Moq Unit of Work

I am new to unit testing and I want to create a test for my search feature. My service layer looks something like:

public class EmployeeService: BaseService, IEmployeeService
{
    public EmployeeService(IUnitOfWork unitOfWork) : base(unitOfWork)
    {
        _employeeRepo = unitOfWork.EmployeeRepository;
    }

    public IEnumerable<Employee> Search(Employee advancedSearch, int[] divisionIds, bool showInactive, int pageSize, int? page)
    {
        return _employeeRepo.Search(advancedSearch, divisionIds, showInactive, pageSize, page);
    }
}

Unit Test:

[Test]
public void SearchShouldFilterActiveEmployees()
{
    var employees = new List<Employee>
    {
        new Employee { EmployeeId = 105, FirstName = "John", LastName = "Smith", IsActive = true },
        new Employee { EmployeeId = 162, FirstName = "John", LastName = "Doe", IsActive = true },
        new Employee { EmployeeId = 3, FirstName = "Jan", LastName = "Doe", IsActive = true }
    };

    var mockUnitOfWork = new Mock<IUnitOfWork>();

    var sut = new EmployeeService(mockUnitOfWork.Object);

    var employeeSearchCriteria = new Employee
    {
        FirstName = "John"
    };

    var employeesReturned = sut.Search(employeeSearchCriteria, null, false, 25, 1);

    Assert.IsTrue(employeesReturned.Count() == 2);
}

I think the issue is with mocking the UnitOfWork. I am getting a Null Reference Exception. How can I moq the UnitOfWork so I can test that the correct number of employees is being returned from the Search.