I have the following xaml (nonessential markup removed in the interest of brevity):

<navigation:Frame x:Name="ContentFrame" >
    <navigation:Frame.UriMapper>
        <uriMapper:UriMapper>
            <uriMapper:UriMapping Uri="/{pageName}" 
                                  MappedUri="/Views/{pageName}.xaml"/>
            <uriMapper:UriMapping Uri="/FMChart/{metricID}/{orgID}" 
                                  MappedUri="/Views/FMChart.xaml?metricID={metricID}&orgID={orgID}"/>
        </navigation:Frame.UriMapper>
    </navigation:Frame.UriMapper>
</navigation:Frame>

I'm creating the HyperLinkButton objects dynamically (in code), like so:

int metricID = 1;
int orgID = 1;

HyperlinkButton button = new HyperlinkButton();
button.Name        = Guid.NewGuid().ToString();
button.TargetName  = "ContentFrame";

// this string doesn't work
string url = string.Format("/FMChart/{0}/{1}", metricID, orgID);

button.NavigateUri = new Uri(url, UriKind.Relative);

When I click the bbutton, the browser renders a blank page, and eventually presents me with a REALLY long stack trace (InvalidOperation exception). If I take the parameters out of th indicated line:

string url = "/FMChart";

...it works as expected (brings up the desired page).

I've also tried the following strings:

/FMChart/{0}&{1}

/FMChart/{0}& amp;{1}

What am I doing wrong?