出现 NSInternalInconsistencyException 的错误，不知如何下手查

Question

*** Assertion failure in -[NSLayoutConstraint constant], /SourceCache/Foundation/Foundation-992/Layout.subproj/NSLayoutConstraint.m:560
2012-10-01 14:01:01.889 RemindMe[1116:907] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '(null)'

我什么都没干。。。viewDidLoad里加了一个button

// button
    self.mybtn = [UIButton buttonWithType:UIButtonTypeCustom];
    self.mybtn.frame = CGRectMake(106, 236, 110, 40);
    [self.mybtn setBackgroundImage:[UIImage imageNamed:@"btnbg.png"] forState:UIControlStateNormal];
    [self.mybtn addTarget:self action:@selector(clicked) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:self.mybtn];

就这么简单的代码，他就报错了。。。求解，怎么回事啊。。。