Exception AttributeError: 'list' object has no attribute 'split' on charts endpoint

[ukanga]

File "./onadata/apps/api/viewsets/charts_viewset.py" in retrieve
  94.                 field_name, xform, fmt, group_by, field_xpath)

File "./onadata/libs/utils/chart_tools.py" in get_chart_data_for_field
  493.             data_view=data_view)

File "./onadata/libs/utils/chart_tools.py" in build_chart_data_for_field
  309.                 choices=grp_choices)

File "./onadata/libs/utils/chart_tools.py" in _use_labels_from_group_by_name
  204.                         get_choice_label(choices, item[truncated_name])

File "./onadata/libs/utils/chart_tools.py" in get_choice_label
  118.                 find_choice_label(choices, name) for name in string.split(" ")

Exception Type: AttributeError at /api/v1/charts/[pk].json
Exception Value: 'list' object has no attribute 'split'
Request information:
GET:
group_by = u'sex,sex'
field_xpath = u'people'

Aha! Link: https://ona.aha.io/features/PROD-593