Fix assert for #388
Fix assert for #388

Summary:
Fixed #388

- [x] All tests pass. Demo project builds and runs.
- [ ] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/391

Differential Revision: D4389116

Pulled By: rnystrom

fbshipit-source-id: 632fd4b4e723387e220589a3be06d59981b8c43c

zhubofei/IGListKit,dshahidehpour/IGListKit,charles0126dev/IGListKit,dshahidehpour/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,Instagram/IGListKit,charles0126dev/IGListKit,Instagram/IGListKit,charles0126dev/IGListKit,zhubofei/IGListKit,dshahidehpour/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit