What's wrong with this observeValueForKeyPath:ofObject:change:context: implementation?

In my UIScrollView subclass, I'm observing frame changes:

[self addObserver:self forKeyPath:@"frame" options:0 context:NULL];

My observeValueForKeyPath:ofObject:change:context: implementation is as follows:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if (object == self && [keyPath isEqualToString:@"frame"]) {
        [self adjustSizeAndScale];
    }
    if ([UIScrollView instancesRespondToSelector:@selector(observeValueForKeyPath:ofObject:change:context:)]) {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context]; // Exception
    }
}

But I get exception with this code:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '<WLImageScrollView: 0x733a440; baseClass = UIScrollView; frame = (0 0; 320 416); clipsToBounds = YES; layer = <CALayer: 0x7346500>; contentOffset: {0, 0}>: An -observeValueForKeyPath:ofObject:change:context: message was received but not handled.
Key path: frame
Observed object: <WLImageScrollView: 0x733a440; baseClass = UIScrollView; frame = (0 0; 320 416); clipsToBounds = YES; layer = <CALayer: 0x7346500>; contentOffset: {0, 0}>
Change: {
    kind = 1;
}
Context: 0x0'

Does it mean UIScrollView implements observeValueForKeyPath:ofObject:change:context: but throws the above exception?

If so, how can I properly implement observeValueForKeyPath:ofObject:change:context: so that I can both handle my interested changes and give superclass a chance to handle its interested changes?