while executing get_SNP_candidates.py and get_tensors gave this error

[siddhantrathore]

Traceback (most recent call last):
File "get_SNP_candidates.py", line 172, in
make_variant_candidates(args)
File "get_SNP_candidates.py", line 103, in make_variant_candidates
pileup[pos][b] += 1
KeyError: 'N'
samtools view: writing to standard output failed: Broken pipe
samtools view: error closing standard output: -1