Understanding UICollectionView insertSections: - ios

Question

I have a UICollectionView in my iOS application. I'd like to use the various insert... methods to add data, primarily for the animations provided by iOS when using these methods (instead of reloadData:).
If I try to add a section, however, I get an exception from iOS. My code is extremely straightforward:
_regions = [NSMutableArray array];
[self.collectionView reloadData];
NSLog (#"Before attempting to add section, collection view has %ld sections", [self numberOfSectionsInCollectionView:self.collectionView]);
[_regions addObject:#"First Region"];
[self.collectionView insertSections:[[NSIndexSet alloc] initWithIndex:0]];
NSLog (#"After attempting to add section, collection view has %ld sections", [self numberOfSectionsInCollectionView:self.collectionView]);
So I add an object to my _regions array, which I use to implement the data source, then I call insertSection. My first NSLog works:
2015-03-24 11:25:39.052 MyApp[63858:17968323] Before attempting to add section, collection view has 0 sections
but then iOS throws an exception:
2015-03-24 11:25:42.082 MyApp[63858:17968323] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of sections. The number of sections contained in the collection view after the update (1) must be equal to the number of sections contained in the collection view before the update (1), plus or minus the number of sections inserted or deleted (1 inserted, 0 deleted).'
I implement numberOfSectionsInCollectionView in pretty straightforward manner:
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
return _regions.count;
}
If I reverse the calls (call insertSections before adding the string to the _regions array), I get a different exception:
2015-03-24 11:33:17.472 MyApp[63965:18001840] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'attempt to insert section 0 but there are only 0 sections after the update'
When should I modify (add an element to) _regions, relative to when I add the section to the UICollectionView?
Thanks so much for any help from experienced UICollectionView programmers . . .