WPF CheckBox TwoWay Binding not working

I have

<DataGridCheckBoxColumn 
     Binding="{Binding Path=Foo, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
 />

And

public bool Foo{ get; set; }

Checking/Unchecking sets Foo, but setting Foo in code does not change the Checkbox state. Any Suggesitons?