Unhelpful error message "Tool definition failed initialization"

I'm trying simple cwl script below name `vcfstat.cwl`
```
#!/usr/bin/env cwl-runner
cwlVersion: v1.0
class CommandLineTool
baseCommand: bcftools stats
inputs:
vcf_file:
type: File
inputBinding:
position: 1
outputs: []
```

Then run `cwl-runner vcfstat.cwl my_file.vcf.gz` and got the error message below.
```
cwl-runner vcfstats.cwl data/my_file.vcf.gz
INFO /usr/local/bin/cwl-runner 1.0.20190815141648
INFO Resolved 'vcfstats.cwl' to 'file:///home/suktitipat/vcfstats.cwl'
ERROR Tool definition failed initialization:
'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte
```