模拟器不显示按钮文本

我正在学习使用XCode 4.5.2构建iPhone应用程序,我发现了一些奇怪的东西.正如您在地址 http://i.stack.imgur.com/purI8.jpg处所看到的,其中一个按钮内的文本未显示在iOS6模拟器中.我还尝试在0和 – 的相同行中移动Enter按钮,但该行的所有三个按钮中的文本都消失了.任何人都知道这个问题的原因是什么以及如何解决？这是代码：

#import "CalculatorViewController.h"
#import "CalculatorBrain.h"

@interface CalculatorViewController()
@property (nonatomic) BOOL userIsInTheMiddleOfEnteringANumber;
@property (nonatomic,strong) CalculatorBrain *brain;
@end

@implementation CalculatorViewController

@synthesize display;
@synthesize userIsInTheMiddleOfEnteringANumber;
@synthesize brain = _brain;

- (CalculatorBrain *)brain
{
    if (!_brain) _brain = [[CalculatorBrain alloc] init];
    return _brain;
}

- (IBAction)digitpressed:(UIButton *)sender
{    
    Nsstring *digit = [sender currentTitle];
    if (self.userIsInTheMiddleOfEnteringANumber) {
        self.display.text = [self.display.text stringByAppendingString:digit];
    } else {
        self.display.text = digit;
        self.userIsInTheMiddleOfEnteringANumber = YES;
    }
}

- (IBAction)enterpressed
{
     [self.brain pushOperand:[self.display.text doubleValue]];
     self.userIsInTheMiddleOfEnteringANumber = NO;
}

- (IBAction)operationpressed:(UIButton *)sender
{
    if (self.userIsInTheMiddleOfEnteringANumber) [self enterpressed];

    Nsstring *operation = [sender currentTitle];
    double result = [self.brain performOperation:operation];
    self.display.text = [Nsstring stringWithFormat:@"%g",result];
}

@end

- (void)setTitle:(Nsstring *)title forState:(UIControlState)state

- (IBAction)operationpressed:(UIButton *)sender{
   ....
   [sender setTitle:[Nsstring stringWithFormat:@"%g",result] forState: UIControlStatenormal];

   // lets assume you want the down states as well:
   [sender setTitle:[Nsstring stringWithFormat:@"%g",result] forState: UIControlStateSelected];
   [sender setTitle:[Nsstring stringWithFormat:@"%g",result] forState: UIControlStateHighlighted];