Func variance with multiple parameters

Tried to something like this in our code but it fails:

Func<Employee, Employee> _myFunc;

void Main()
{
    Func<Employee, Employee> test1  = _myFunc;//Ok
    Func<Employee, Person> test2  = _myFunc;//Ok
    Func<Person, Employee> test3 = _myFunc;//Fails
    Func<Person, Person> test4  = _myFunc;//Fails
}

public class Person { }
public class Employee : Person { }

The last two cases give this error:

Cannot implicitly convert type System.Func<Employee, Employee> to System.Func<Person, Employee>. An explicit conversion exists (are you missing a cast?)

Any idea why?