MKAnnotationView drawRect: not getting called

I've implemented a custom annotation derived from MKAnnotation called ContainerAnnotation and a custom annotation view derived from MKAnnotationView with a drawRect: method called ContainerAnnotationView. For some reason the drawRect: method is not getting called and I can't figure out why.

Here's the source code for my annotation view.

ContainerAnnotationView.h:

@interface ContainerAnnotationView : MKAnnotationView
{
}

@end

ContainerAnnotationView.m:

@implementation ContainerAnnotationView

- (void) drawRect: (CGRect) rect
{
    // Draw the background image.
    UIImage * backgroundImage = [UIImage imageNamed: @"container_flag_large.png"];
    CGRect annotationRectangle = CGRectMake(0.0f, 0.0f, backgroundImage.size.width, backgroundImage.size.height);
    [backgroundImage drawInRect: annotationRectangle];

    // Draw the number of annotations.
    [[UIColor whiteColor] set];
    UIFont * font = [UIFont systemFontOfSize: [UIFont smallSystemFontSize]];
    CGPoint point = CGPointMake(2, 1);
    ContainerAnnotation * containerAnnotation = (ContainerAnnotation *) [self annotation];
    NSString * text = [NSString stringWithFormat: @"%d", containerAnnotation.annotations.count];
    [text drawAtPoint: point withFont: font];
}

@end

From my view controller:

- (MKAnnotationView *) mapView: (MKMapView *) mapView viewForAnnotation: (id <MKAnnotation>) annotation
{

    if ([annotation isKindOfClass: [ContainerAnnotation class]])
    {
        ContainerAnnotationView * annotationView = (ContainerAnnotationView *)[_mapView dequeueReusableAnnotationViewWithIdentifier: _containerMapAnnotationId];
        if (annotationView == nil)
        {
            annotationView = [[[ContainerAnnotationView alloc] initWithAnnotation: annotation reuseIdentifier: _containerMapAnnotationId] autorelease];     
            annotationView.centerOffset = CGPointMake(0, -17.5);
            annotationView.rightCalloutAccessoryView = [UIButton buttonWithType: UIButtonTypeDetailDisclosure];
            annotationView.canShowCallout = YES;
        }
        annotationView.annotation = annotation;

        return annotationView;
    }
    // etc...
}

I have other annotations that use a vanilla MKAnnotation with an image that work fine. I also have another custom annotation view that doesn't implement drawRect: that works fine. Any idea what I'm doing wrong here?

The problem turned out to be that my drawRect: method was never being called because the frame was not set to a non-zero size. Adding an initWithAnnotation: method to do this solved the problem.

- (id) initWithAnnotation: (id <MKAnnotation>) annotation reuseIdentifier: (NSString *) reuseIdentifier
{
    self = [super initWithAnnotation: annotation reuseIdentifier: reuseIdentifier];
    if (self != nil)
    {
        self.frame = CGRectMake(0, 0, 30, 30);
        self.opaque = NO;
    }
    return self;
}

Did you call setNeedsDisplay for this view subclass anywhere? (Just after you make this view visible is a good place.)