Point is no longer accepted when I use the property UpdateSourceTrigger

Question

I used the property "UpdateSourceTrigger = PropertChanged". I am binding on a double. However, I can not do a '.' anymore, this is simply filtered out / deleted. How can I fix this an continue to use the property?

My Property:

private double textBoxZahl;
public double TextBoxZahl 
{ 
    get => textBoxZahl; 
    set
    {
        textBoxZahl = value;
        OnPropertyChanged();
    }
}

My TextBox:

<TextBox Margin="1,1,1,1" BorderThickness="1" Grid.ColumnSpan="2" Grid.Column="1" Grid.Row="0" Width="auto" x:Name="_NameBox" Background="WhiteSmoke" Text="{Binding ElementName=MaterialList, Path=SelectedItem.TextBoxZahl, UpdateSourceTrigger=PropertyChanged}"/>

My binding works fine, but not if I try to use a dot.