IMultiValueConverter converter problem???

Question

Hello, I am tryng to write a multibinding converter for a navigation model where I load pages into a frame when any listboxitem is selected from two listboxes. At the same time, I need to be able to navigate with Back/Forward buttons from the navigation class and being able to show listboxitem in selected state if its UriSource is loaded into the frame. The coverter I have can update urisources from two sources-listboxes in the frame but cannot switch listboxitem in selected state. I switch listboxitem into selected state on ConvertBack portion of the converter. I did something wrong, I am always getting errors on a built. Please let me know if it possible to achieve what I m trying to do. Thank you in advance.
The below is the MultiBindConverter code:

 

public class MultiBindConverter : IMultiValueConverter
{
#region IMultiValueConverter Members
public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if (values[0] != null)
{
if (values[1] != null)
{
return new Uri(values[1].ToString(), UriKind.RelativeOrAbsolute);
}
return new Uri(values[0].ToString(), UriKind.RelativeOrAbsolute);
}
return null;
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
{ 
if (value != null)
{
var uri = (Uri)value;
var uriString = uri.OriginalString;
if (uri.OriginalString.Contains(";component/"))
{
uriString = uriString.Substring(uriString.IndexOf("/") + 1);
}
return new object[] {uriString, uriString}; 
}
//return Binding.DoNothing;
} 
#endregion IMultiValueConverter Members
} 
 

XAML:

 

<Frame Grid.Column="2" x:Name="ContentFrame" JournalOwnership="OwnsJournal" NavigationUIVisibility="Visible">
<Frame.Source>
<MultiBinding Converter="{StaticResource MultiBindConverter}">
<Binding Path="SelectedValue" ElementName="Nav_ListBox" Mode="TwoWay" />
<Binding Path="SelectedValue" ElementName="SublevelListbox" Mode="TwoWay" />
</MultiBinding>
</Frame.Source>
</Frame>
 

 
Thank you.