AttributeError: 'NoneType' object has no attribute 'get'

[gijzelaerr]

$ .virtualenv/bin/cwltool --debug ndppp_prep_cal.cwl --msin data/L591513_SB000_uv.MS
.virtualenv/bin/cwltool 1.0.20171017195544
Resolved 'ndppp_prep_cal.cwl' to 'file:///home/gijs/prefactor-cwl/ndppp_prep_cal.cwl'
I'm sorry, I couldn't load this CWL file.  The error was:
Traceback (most recent call last):
  File "/home/gijs/prefactor-cwl/.virtualenv/local/lib/python2.7/site-packages/cwltool/main.py", line 818, in main
    skip_schemas=args.skip_schemas)
  File "/home/gijs/prefactor-cwl/.virtualenv/local/lib/python2.7/site-packages/cwltool/load_tool.py", line 214, in validate_document
    _convert_stdstreams_to_files(workflowobj)
  File "/home/gijs/prefactor-cwl/.virtualenv/local/lib/python2.7/site-packages/cwltool/load_tool.py", line 85, in _convert_stdstreams_to_files
    if out.get('type') == streamtype:
AttributeError: 'NoneType' object has no attribute 'get'

CWL file
https://github.com/EOSC-LOFAR/prefactor-cwl/blob/393a99922fe84e161df1973bb30607295c0f9672/ndppp_prep_cal.cwl

[mr-c]

I've update the original post to reference to the exact file that cause the unhelpful error