title: calling setCoordinate on MKPlacemark crashes app with 'unrecognized selector'
body: <p>I have a MKMapView i'm adding a single MKPlacemark to to represent the location of a building the user has just selected.  The user can only select one building at a time, and I simply want to move the placemark to the new building when they select a new building.  On the first building they select, it works fine and places a pin on the map.  When I try and call <code>setCoordinate</code> on the placemark to update the position of the marker when they select a new building though, I get <code>-[MKPlacemark setCoordinate:]: unrecognized selector sent to instance</code></p>
  
  <p>In MyViewController.h I have:</p>
  
  <pre><code>@property (nonatomic, strong)MKPlacemark *selectedBuildingPlacemark;
  </code></pre>
  
  <p>In MyViewController.m</p>
  
  <pre><code>@synthesize selectedBuildingPlacemark;
  
  ...
  
  if (self.selectedBuildingPlacemark == nil) {
          self.selectedBuildingPlacemark = [[MKPlacemark alloc] initWithCoordinate:myCoord addressDictionary:nil];
          [mapView addAnnotation:self.selectedBuildingPlacemark];
      }
      else {
          [self.selectedBuildingPlacemark setCoordinate:myCoord];
      }
  </code></pre>
  
  <p>I thought MKPlacemark conformed to MKAnnotation and should therefore implement <code>setCoordinate</code>.  Can someone show me the error of my ways?</p>