ios – UIPickerView导致崩溃

我已经实现了所有委托方法但收到此错误：

2013-01-15 13:57:56.176 tracker[16142:c07] *** Assertion failure in -[UITableViewRowData rectForRow:inSection:],/SourceCache/UIKit_Sim/UIKit-2372/UITableViewRowData.m:1630
2013-01-15 13:57:56.177 tracker[16142:c07] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException',reason: 'request for rect at invalid index path (<NSIndexPath 0x918c700> 2 indexes [0,0])'
*** First throw call stack:
(0x1c97012 0x10d4e7e 0x1c96e78 0xb6af35 0x2050c0 0xc892e 0x457fc6 0x457eba 0x7ba5d 0x7e03b 0x2e689a 0x2e59db 0x2e711f 0x2e9d6d 0x2e9cec 0x2e1a68 0x4efc2 0x4f4a3 0x2d3aa 0x1ecf8 0x1bf2df9 0x1bf2ad0 0x1c0cbf5 0x1c0c962 0x1c3dbb6 0x1c3cf44 0x1c3ce1b 0x1bf17e3 0x1bf1668 0x1c65c 0x29bd 0x28e5 0x1)
libc++abi.dylib: terminate called throwing an exception

以下是实现的委托方法：

- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)thePickerView {

    return 1;
}

- (NSInteger)pickerView:(UIPickerView *)thePickerView numberOfRowsInComponent:(NSInteger)component { 
    return [classes count];
}


- (NSString *)pickerView:(UIPickerView *)thePickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component {
    return [classes objectAtIndex:row];
}

- (void)pickerView:(UIPickerView *)thePickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component {

    NSLog(@"Selected Class: %@. Index of selected color: %i",[classes objectAtIndex:row],row);
}

‘classes’是一个NSMutableArray.

以下是视图的连线方式：

受控视图称为MemberViewController.

我的接口声明包含以下内容：

@interface MemberViewController : UIViewController <UIPickerViewDataSource,UIPickerViewDelegate>

我究竟做错了什么？

谢谢.