fix #88
fix #88

Summary: Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/89

Differential Revision: D17950095

Pulled By: ppwwyyxx

fbshipit-source-id: 63a58dab7e74cf94b64a906965d55dfa8012ed9b

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2