WPF Extended Toolkit CheckCombobox Binding problem

Question

I have a CheckComboBox from WPF Extended Toolkit and SelectedItemsOverride does not trigger my property's setter and I don't know why. What am I doing wrong?

My XAML:

<xctk:CheckComboBox ItemsSource="{Binding VeterinaryChoices}"
                    IsSelectAllActive="True" 
                    DisplayMemberPath="FullName" 
                    ValueMemberPath="EmployeeID" 
                    SelectedItemsOverride="{Binding SelectedVeterinaries, 
                          ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}"/>

Property in ViewModel:

 public ObservableCollection<Employee> SelectedVeterinaries
    {
        get
        {
            return _selectedVeterinaries;
        }
        set
        {
            _selectedVeterinaries = value;
            OnPropertyChanged("SelectedVeterinaries");
        }
    }

The context is set properly because my other properties are binded as expected.