vp-comparefits broken by something dataset_inputs related

Created by: voisey

When I run vp-comparefits on master I get the following:

[INFO]: Starting NNPDF fit comparison:
    Current fit: 130920-nnpdf40_jcm_iterated
    Reference fit: 130920-nnpdf40_jcm
    Title: Comparison between 130920-nnpdf40_jcm_iterated and 130920-nnpdf40_jcm
    Author: Cameron Voisey
    Keywords: ['nnpdf40', 'best_average']
    Current fit label: Iteration
    Reference fit label: Baseline
    Thcovmat if present: False
[WARNING]: Output folder exists: /Users/CameronVoisey/nnpdfgit/nnpdf/output Overwriting contents
[WARNING]: old fit found, falling back to old behaviour
[ERROR]: Failed processing key experiments.
[ERROR]: Bad configuration encountered:
Could retrieve element dataset_inputs from namespace. No such key

Any ideas why?