在注释视图上使用canShowCallout时出现mkmapkit异常

我正在尝试使用一个非常简单的自定义地图注释视图和标注-创建注释视图时,只需将uiImageView作为子视图添加到其自身。那很好。

但是,当我在注释视图上调用CanShowCallout时,在返回视图后立即在MapKit中抛出异常。堆栈的结尾如下:

#0  0x94e964e6 in objc_exception_throw
#1  0x01e26404 in -[MKOverlayView _addViewForAnnotation:]
#2  0x01e22037 in -[MKOverlayView _addViewsForAnnotations:animated:]
#3  0x01e1ddf9 in -[MKOverlayView showAddedAnnotationsAnimated:]
#4  0x01df9c0e in -[MKMapView _showAddedAnnotationsAndRouteAnimated:]
#5  0x01e0371a in -[MKMapView levelView:didLoadTile:]

我对注释的看法很简单:

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation
{
    if ( ! [annotation isKindOfClass:[MyAnnotation class]] )
        return nil;

    MyAnnotationView *useView = (MyAnnotationView *)[myMapView dequeueReusableAnnotationViewWithIdentifier:@"resuseview"];
    if ( useView == nil )
    {
        useView = [[[MyAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"resuseview"] autorelease];
        useView.canShowCallout = YES;  // if commented out view appears just fine
    }
    else
    {   useView.annotation = annotation;  }

    return useView;
}

如代码中所述,注释视图工作正常-直到我添加canShowCallout,然后在地图第一次获得视图时崩溃。