Wrong collectionViewContentSize

[ftp27]

I use UICollectionView with NHBalancedFlowLayout inside another UICollectionView because I must get the height of content in NHBalancedFlowLayout by this code:

[((NHBalancedFlowLayout *) self.filesList.collectionViewLayout) prepareLayout];
CGFloat collectionSize = [((NHBalancedFlowLayout *) self.filesList.collectionViewLayout) collectionViewContentSize].height;

But this function returns 0. How can I get the real size of content?

[ftp27]

I located this issue. Content gets wrong height when one of sections has 0 count of cells.