unrecognized selector sent to instance
===

I'm attempting to initialize an array and add a chapter object to an array of book objects, but it crashes with the error:

2012-07-25 21:41:01.503 Project1[2364:f803] -[__NSCFString arrayOfChapters]: unrecognized selector sent to instance 0x6ad80b0
2012-07-25 21:41:01.505 Project1[2364:f803] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString arrayOfChapters]: unrecognized selector sent to instance 0x6ad80b0'

My code:

            Chapter *myChapter = [[Chapter alloc]init];
            myChapter.pageCount = self.TextField2.text;
            myChapter.chapterTitle = self.TextField1.text;
            
            if(!currentBook.arrayOfChapters)
            {
                currentBook.arrayOfChapters = [[NSMutableArray alloc]init];
            }
            currentBook = [books objectAtIndex:segControl.selectedSegmentIndex];
            
            [currentBook.arrayOfChapters addObject:myChapter];

I think the code is correct, is there something set up wrong with my project?  I believe it's the initialization which is causing the actual crash, but there isn't anything non-standard there.