[NSCFNumber isEqualToString:]: unrecognized selector sent to instance

Alright, I'm having the following common problem

[NSCFNumber isEqualToString:]: unrecognized selector sent to instance

but this time I'm not sure how to fix it.

Here's the declaration in viewDidLoad:

- (void)viewDidLoad {

    [super viewDidLoad];

    NSMutableArray *tempHours = [NSMutableArray array];
    for (int i = 0; i < 12; i++) { 
        [tempHours addObject:[NSNumber numberWithInt:(i+1)]];
    }
    self.hours = tempHours; // 'hours' is a synthesized NSArray property of the ViewController
    [tempHours release];

  // two more similar array declarations here

}

Here's the code method of the UIPickerView where stuff breaks (e.g., the if statement)

- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component {

    NSString *stringIndex = [NSString stringWithFormat:@"Row #%d", row];

    if(component == 0) {
        return stringIndex = [self.hours objectAtIndex:row];
    }

    // more code for other components (of the same form) here

    return stringIndex;
}

I think I need my NSArray of NSNumber objects to be type-casted as strings. How do I do that properly with that statement:

stringIndex = [self.hours objectAtIndex:row];

Thanks!