How do I validate Select with Default Value?

[AlexNek]

I have tried several validators without success. What is the correct way, as I have already set the corrected value and don't want to change it?

<Validation Validator="@???">
   <Field Horizontal>
       <FieldLabel ColumnSize="ColumnSize.IsFull.OnTablet.Is3.OnDesktop">Frequency</FieldLabel>
       <FieldBody ColumnSize="ColumnSize.IsFull.OnTablet.Is9.OnDesktop">
           <Select TValue="string" @bind-SelectedValue="@_frequency">
               <SelectItem Value="@DefDaily">Daily</SelectItem>
               <SelectItem Value="@DefWeekly">Weekly</SelectItem>
               <SelectItem Value="@DefMonthly">Monthly</SelectItem>
           </Select>
       </FieldBody>
   </Field>
</Validation>
...
_frequency = DefDaily;

Validation only works when I select something.

[AlexNek]

Solved, was my error