Why does setting the diameter cause the size to go to {0,0}?

gistfile1.m

@implementation ExpandButton
{
  NSLayoutConstraint *_widthConstraint;
}

- (id)initWithFrame:(CGRect)frame
{
  CGFloat smallerDim = MIN(frame.size.height, frame.size.width);
	
    self = [super initWithFrame:frame];
    if (self)
	{
		// stop autoresizing mask from interfering
		self.translatesAutoresizingMaskIntoConstraints = NO;
		
		// width and height should always be equal
		NSLayoutConstraint *aspectConstraint = [NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self attribute:NSLayoutAttributeHeight multiplier:1.0 constant:0];
		[self addConstraint:aspectConstraint];
		
		// specify a constraint for the width
		_widthConstraint = [NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:smallerDim];
		[self addConstraint:_widthConstraint];
    }
    return self;
}

- (void)setDiameter:(CGFloat)diameter
{
	if (_diameter == diameter)
	{
		return;
	}
	
	_widthConstraint.constant = _diameter;
}

@end