WPF Bind checkbox to bool?

I have a WPF checkbox binded to ViewModel nullable boolean property. I am setting this property to false or true in Constructor, to avoid Interminent state but no matter what I do the Initial state of checkbox stays grayed. The binding working just fine since once I change the state by clicking the checkbox on UI I am getting controls values (true/false). Any Ideas?

XAML

 <CheckBox Margin="0,4,0,3" VerticalAlignment="Center" Content="Mutual"
                                IsChecked="{Binding MutualChb}" />

ViewModel

 public ContstrutorViewModel()
        {
            MutualChb = true;
        }


      private bool? _mutualChb;
        public bool? MutualChb
        {
            get { return _mutualChb; }
            set { 
                  _mutualChb = value; 
                  _mutualChb = (_mutualChb != null ) ? value : false;
                  OnPropertyChanged("MutualChb"); }
        }