Add validation to InputSelect with default placeholder string

I´m using an InputSelect like this:

<InputSelect @bind-Value="ExampleName"><option>---</option>    @foreach (var item in itemList)    {<option>@item.Name</option>    }</InputSelect><ValidationMessage For="@(() => ExampleName)" />

The validation is implemented like this:

[Required(ErrorMessage = "Custom error message")]public string ExampleName { get; set; }

If I select nothing from the input select, the validation works fine, because ExampleName does not contain anything and thus the [Required] attribute isn´t fulfilled. But as soon as I select a valid option, and select the placeholder (---) again, the validation tells me, that the input is valid. Of course it is, because --- is a string, but I don´t want it to be valid.

How can this option be included as non-valid?