Wpf: adding new row to datagrid, but there's no default value

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C#

WPF

collections

DataGrid

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My DataGrid is bound to an ObservableCollection in my ViewModel. I add new rows via a command that looks like this:

public void AddItem() {
    MyObservableCollection.Add(new MyClass());
}

How can I set default value for a DataGridComboBoxColumn when I create new rows, because as is - it turns up empty, which mess up my bindings.
If I could, for example, bind it to Item at index 1, or something, that'd be great.

The XAML for the DataGrid and the DataGridComboBoxColumn looks like this:

<DataGrid
    AutoGenerateColumns="False"
    ItemsSource="{Binding MyObservableCollection}">

     <DataGridComboBoxColumn
         ItemsSource="{Binding Source={StaticResource MyObservableCollection}}"
         SelectedValueBinding="{Binding PropertyName1}"
         DisplayMemberPath="PropertyName1"
         SelectedValuePath="PropertyName1" />

</DataGrid>

What I have tried:

I tried setting the TargetNullValue and FallBackValue, but it did not work. I also tried setting the DefaultValue when instantiating the new row, but it did not work. I would prefer a MVVM-solution, but am also fine with a code-behind hack..