How to debug BaseMap Selection (Silverlight)

[ShaningYu]

In my SL project, there is a stackpanel for basemap selection, in which there are 3 radio buttons for Streets, Topo and Imagery, respectively.  It used to work well.  Now, only the Streets map display initially and the selection of basemap losses its function.  If I click a another basemap, the error (shown below) pops out.  How can I debug on it?  Thanks.

[ShaningYu]

Problem found - I changed Target Name to make it becomes consistent with the map's definition. Thanks for your review.