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];
}