Issue title: ERROR: pycocotools unable to run: list index out of range
 Issue body: I meet a problem，how can i solve it?

COCO mAP with pycocotools... saving detections_val2017__results.json...
ERROR: pycocotools unable to run: list index out of range
Optimizer stripped from runs/exp5/weights/last.pt, 14.7MB
Optimizer stripped from runs/exp5/weights/best.pt, 14.7MB

 Comments: 
 Comment 0: Hello @Lie-huo, thank you for your interest in our work! Please visit our [Custom Training Tutorial](https://github.com/ultralytics/yolov5/wiki/Train-Custom-Data) to get started, and see our [Jupyter Notebook](https://github.com/ultralytics/yolov5/blob/master/tutorial.ipynb) <a href="https://colab.research.google.com/github/ultralytics/yolov5/blob/master/tutorial.ipynb"><img src="https://colab.research.google.com/assets/colab-badge.svg" alt="Open In Colab"></a>, [Docker Image](https://hub.docker.com/r/ultralytics/yolov5), and [Google Cloud Quickstart Guide](https://github.com/ultralytics/yolov5/wiki/GCP-Quickstart) for example environments.
  
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- **Cloud-based AI** systems operating on **hundreds of HD video streams in realtime.**
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 Comment 1: @Lie-huo pycocotools is an add-on you can use with COCO data. It will try and run anytime your data file is coco.yaml.
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