Mutating method sent to immutable object closes app

In my code ,when the user completes a level, it unlocks the next level (out of a total of 15 levels).

However, I have noticed that, when I complete the level, first time, it works, however, when I go back to it and try again, it crashes with this error;

[__NSCFArray replaceObjectAtIndex:withObject:]: mutating method sent to immutable object

The code part is this;

//Unlock Next Level
                if (levelNumber != 15) {
                    [m_appDelegate.levels replaceObjectAtIndex:levelNumber withObject:[NSNumber numberWithBool:NO]];
                    [m_appDelegate saveLevels];
                }

If I remove this line;

[m_appDelegate.levels replaceObjectAtIndex:levelNumber withObject:[NSNumber numberWithBool:NO]];

Then the app does not crash, but of course, it does not unlock any further levels.

Some references below that may help;

int levelNumber;
@property (nonatomic, readwrite) int levelNumber;


- (void) actionLockedLevel:(id)sender {
    selectedLevel = ((CCNode*)sender).tag;
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Level Locked" message:@"Level is locked. Pass the previous level first." delegate:nil cancelButtonTitle:@"No" otherButtonTitles:nil];
    [alert show];
}

- (void) actionUnlockedLevel:(id)sender {

    CCScene *scene = [CCScene node];
    TimedLevel *layer = [TimedLevel node];
    layer.levelNumber = ((CCNode*)sender).tag;
    [scene addChild:layer];


    [[CCDirector sharedDirector] replaceScene:scene];
}

In AppDelegate.m, levels are called in this;

- (void) loadLevels {
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    if ([defaults objectForKey:@"LEVELS"] == nil) {

        levels = [[NSMutableArray alloc] initWithObjects:
                  [NSNumber numberWithBool:NO],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],
                  [NSNumber numberWithBool:YES],nil];

    } else {
        self.levels = [defaults objectForKey:@"LEVELS"];
    }

    if ([defaults objectForKey:@"LEVELS_PIZZAS"] == nil) {

        levelsPizzas = [[NSMutableArray alloc] initWithObjects:
                        [NSNumber numberWithInteger:200],
                        [NSNumber numberWithInteger:250],
                        [NSNumber numberWithInteger:300],
                        [NSNumber numberWithInteger:350],
                        [NSNumber numberWithInteger:400],
                        [NSNumber numberWithInteger:450],
                        [NSNumber numberWithInteger:500],
                        [NSNumber numberWithInteger:550],
                        [NSNumber numberWithInteger:600],
                        [NSNumber numberWithInteger:650],
                        [NSNumber numberWithInteger:700],
                        [NSNumber numberWithInteger:800],
                        [NSNumber numberWithInteger:900],
                        [NSNumber numberWithInteger:1000],
                        [NSNumber numberWithInteger:1100],nil];

    } else {

        self.levelsPizzas = [defaults objectForKey:@"LEVELS_PIZZAS"]; 
    }

    if ([defaults objectForKey:@"COLLECTED_PIZZAS"] == nil) {
        self.nCurClickAmounts = 0;
    } else {
        self.nCurClickAmounts = [[defaults objectForKey:@"COLLECTED_PIZZAS"] intValue];
    }
    [defaults synchronize];
}

- (void) saveLevels {
    [[NSUserDefaults standardUserDefaults] setObject:levels forKey:@"LEVELS"];
    [[NSUserDefaults standardUserDefaults] setObject:levelsPizzas forKey:@"LEVELS_PIZZAS"];
    [[NSUserDefaults standardUserDefaults] setObject:[NSNumber numberWithInt:nCurClickAmounts] forKey:@"COLLECTED_PIZZAS"];
    [[NSUserDefaults standardUserDefaults] synchronize];
}