How to Bind a nullable bool in VM to RadRadioButton

[Patrick]

I am struggling with how I go about this?  In my VM I have a nullable bool property that I want to bind to a Yes/No(true/false) RadRadioButton.  Can someone point me in the right direction?  

public bool? IsSDS { get; set; }

<telerik:Label Content="Self Directed Support" />
<StackPanel Orientation="Horizontal">
     <telerik:RadRadioButton x:Name="SelfDirectedSupportYes" Content="Yes" />
     <telerik:RadRadioButton x:Name="SelfDirectedSupportNo" Content="No" />
</StackPanel>

[Patrick]

for reference to others I got an answer here --> StackOverflow