iOS Setters and Getters and Underscored Property Names

So I have a NSString property named description, defined as follows:

@property (strong, nonatomic) NSMutableString *description;

I'm able to refer to it as _description when I define the getter, as follows:

- (NSString *)description
{
    return _description;
}

However, when I define a setter, as follows:

-(void)setDescription:(NSMutableString *)description
{
    self.description = description;
}

It breaks _description from the aforementioned getter (undeclared identifier). I know I can probably just use self.description instead, but why does this happen?