FluentValidation, validate a property based on another properties value

Question

This is my model:

public class StaffMemberTask
{
    public int StaffMemberTaskId { get; set; }
    public string TaskDescription { get; set; }
    public Initiator InitiateFrom { get; set; }
    public DateTime? StartFromDate { get; set; }
}

public enum Initiator
{
    StartDate,
    DateOfBirth,
    SpecificDate,
    Birthday
}

I want to validate that StartFromDate is not null (or maybe DateTime.Min) if InitiateFrom == Initiator.SpecificDate.

I have tried these combinations but I just cannot get the validation to trigger on StartFromDate:

RuleFor(x => x.StartFromDate).NotNull().DependentRules(() =>
{
    RuleFor(x => x.InitiateFrom).Equals(Initiator.SpecificDate);
});

RuleFor(x => x.StartFromDate).NotNull().When(x => x.InitiateFrom == Initiator.SpecificDate);

RuleFor(p => p.StartFromDate).Cascade(CascadeMode.Stop).NotNull().When(x => x.InitiateFrom == Initiator.SpecificDate);

Does the fact I'm evaluating against an enum in anyway affect what I'm doing because I cannot understand why one of the above validation rules wouldn't have worked.