You will need to update Garmin MapSource on your PC. Restart your PC and see if the problem persists.

 http://www8.garmin.com/support/download_details.jsp?id=209

If you still have issues with MapSource, then rather use Garmin BASECAMP and Garmin MapInstall, installation instructions below:

• Garmin BASECAMP is an application which enables you to view Garmin compatible maps on your Windows PC and Macintosh, do route planning,  make waypoints and create tracks and routes between your PC/Mac and your Garmin GPS device. This is an application from Garmin and it will replace MapSource one day.
• Garmin MapInstall is an application which enables you to transfer the map to your GPS unit. You will use this application with Garmin BASECAMP.