Use VS string resources in XAML

Hi, I was able to create a ribbon button with the follwing line...
<ribbon:Button Label="Open" Command="MyAppCommands.Open"/>

Now I am trying to use the VS's string resources feature (Resources.resx) to replace the Label (for the localization purpose). I replaced the above code with the following line...
<ribbon:Button Label="{x:Static properties:Resources.MyOpenString}" Command="MyAppCommands.Open"/>
But then I got this error...
"Unknown build error, 'Key cannot be null, Parameter name: key Line xx Position xx.'"

Can I refer the resource in the XAML like that? Any idea?
Thanks!

-Maggie.