Expression is not assignable

I have an object (a custom UIView) in which I have this property in the public API:

@property (nonatomic) CGPoint theOrigin;

then, in the implementation file I have:

@synthesize theOrigin = _theOrigin;

also in the implementation file I have the method:

-(void)setDefaultOrigin {

_theOrigin.x = self.bounds.origin.x + self.bounds.size.width/2;

_theOrigin.y = self.bounds.origin.y + self.bounds.size.height/2;

}

which doesn't generate any compiler complaints but if I try to code the above method

like this:

-(void)setDefaultOrigin {

self.theOrigin.x = self.bounds.origin.x + self.bounds.size.width/2;

self.theOrigin.y = self.bounds.origin.y + self.bounds.size.height/2;

}

I get a complaint on both lines of the method stating that the "Expression is not assignable"

and it won't allow me to build. I am not understanding why I can't code it the second way.