pycocotoools mistakes

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File "/home/md/.local/lib/python3.6/site-packages/pycocotools-2.0-py3.6-linux-x86_64.egg/pycocotools/coco.py", line 439, in annToMask
rle = self.annToRLE(ann)
File "/home/md/.local/lib/python3.6/site-packages/pycocotools-2.0-py3.6-linux-x86_64.egg/pycocotools/coco.py", line 424, in annToRLE
rles = maskUtils.frPyObjects(segm, h, w)
File "pycocotools/_mask.pyx", line 292, in pycocotools._mask.frPyObjects
IndexError: list index out of range

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could u tell me why this happend?
i trained on my own datasets