user cannot add row on datagrid bound to observablecollection

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C#

WPF

Can anyone tell why this datagrid doesn't show the new row, row?

public ObservableCollection<string> PasswordGuesses { get { return _passwordGuesses ?? (_passwordGuesses = new ObservableCollection<string>() { "password" }); } }

<DataGrid Grid.Row="0" AutoGenerateColumns="False" ItemsSource="{Binding PasswordGuesses}" CanUserAddRows="True">
    <DataGrid.Columns>
        <DataGridTextColumn Header="Password" Binding="{Binding .}" MinWidth="150"/>
        <!--<DataGridTemplateColumn Header="Operations">
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <Button Content="-" Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=DataGrid}, Path=DataContext.DeletePasswordCommand}" CommandParameter="{Binding .}" />
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>-->
    </DataGrid.Columns>
</DataGrid>