Iphone – ‘-[NSURL initFileURLWithPath:]: nil string parameter’ when using CoreData

I have some model classes which I wanted to store with the CoreData framework. I have created the xcdatamodel file and added these already existing classes to the CoreData model file. I have also copied the code responsible for managing the context, etc. from the core data project template from the XCode.

Now I have the following code (it was copied, only the file name was changed to HistoryDataModel):

- (NSManagedObjectModel *)managedObjectModel {

   if (managedObjectModel != nil) {
       return managedObjectModel;
   }
   NSString *modelPath = [[NSBundle mainBundle] pathForResource:@"HistoryDataModel"  ofType:@"momd"];
   NSLog(@"Model path:", modelPath);
   NSURL *modelURL = [NSURL fileURLWithPath:modelPath];
   managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];    
   return managedObjectModel;
}

Now when I run this code and try to access data in the CoreData I'm getting the following error:

* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[NSURL initFileURLWithPath:]: nil string parameter'

and it happens when

NSString *modelPath = [[NSBundle mainBundle] pathForResource:@"HistoryDataModel" ofType:@"momd"];

is being executed (the model Path is an empty string).

Any ideas what I might have done wrong?
Thanks!