error came out when executing MappingQC

[Jiaming6666]

I was comparing 2 samples but stuck in step 1. Everything seems fine until Quality control.
What do you think the problem is? Thanks.
`Runtime.totalMemory()=1073217536
Convert chrM.sam to chrM.bam for D87
Convert chrM.sam to chrM.bam for T87
Draw TSS enrichment plot for D87
Draw TSS enrichment plot for T87
Draw fragment distribution plot for D87
Draw fragment distribution plot for T87
Quality control D87
Quality control T87

Traceback (most recent call last):
File "atac-pipe.py", line 703, in
main()
File "atac-pipe.py", line 690, in main
if options.MappingQC: MappingQC()
File "atac-pipe.py", line 283, in MappingQC
pool.map(MappingQC_6, sn)
File "/zhaowei/YJM/miniconda3/envs/atac-pipe/lib/python2.7/multiprocessing/pool.py", line 253, in map
return self.map_async(func, iterable, chunksize).get()
File "/zhaowei/YJM/miniconda3/envs/atac-pipe/lib/python2.7/multiprocessing/pool.py", line 572, in get
raise self._value
AttributeError: 'NoneType' object has no attribute 'group'`