How do I get a WPF validation rule to fire for an empty text box?

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XAML

WPF

validation

I am unable to get a WPF ValidationRule to detect an empty TextBox.

My cut-down code is as follows:

public class NonEmptyStringValidator : ValidationRule
{
    public override ValidationResult Validate( object value, CultureInfo cultureInfo )
    {
        ValidationResult validationResult = new ValidationResult( false, "Value cannot be empty." );
        if( value != null )
        {
            string valueAsString = value as string;
            if( valueAsString.Length > 0 )
                validationResult = ValidationResult.ValidResult;
        }
        return validationResult;
    }
}

My cut-down XAML is as follows:

<TextBox 
    <TextBox.Text>
        <Binding Path="SelectedUser.Username" UpdateSourceTrigger="LostFocus">
            <Binding.ValidationRules>
                <u:NonEmptyStringValidator/>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

A breakpoint placed inside the Validate() function reveals that the code is not run until there is something in the TextBox. Is there a way to get it to work to detect an empty string?

What I have tried:

I have tried various values for UpdateSourceTrigger.

I have also tried hooking into the LostFocus event for the TextBox and writing code to handle this manually. This gets very messy because one thing I need to do is keep focus on the TextBox until is has something in it, and calling Focus() seems to cause the LostFocus event to fire again, which is something else I don't understand.