table: Cannot read property 'className' of undefined

I get this error and it’s located in the utils.js file:

function splitProps(_ref2) {
  var className = _ref2.className, //<---here
      style = _ref2.style,
      rest = _objectWithoutProperties(_ref2, ['className', 'style']);

  return {
    className: className,
    style: style,
    rest: rest
  };
}

I understand that it has something to do with my column headers, so here is (sort of) my columns definition:

columns: [
      {
        header: 'Sections',
        columns: [{
          header: 'Section',
          accessor: 'sectionName',
          sortable: false
        }, {
          header: 'Sub-Section',
          accessor: 'subSectionName',
          sortable: false
        }]
      },
      {
        header: 'KPIs',
        columns: [{
          header: 'KPI',
          accessor: 'kpiName_1',
          minWidth: 200,
          sortable: false,
          render: props => <span>{props.row && props.row.kpiName_2 ? '' : props.value}</span>
        }, {
          header: 'Sub-KPI',
          accessor: 'kpiName_2',
          minWidth: 140,
          sortable: false,
          render: props => <span>{props.row && props.row.kpiName_3 ? '' : props.value}</span>
      //more columns like that
        }]
      },
      {
        header: 'Details',
        columns: [{
          // more columns here
          header: 'NO',
          accessor: 'kpiNumber',
          render: rowInfo => <span>{rowInfo.value}</span>,
          width: 50,         
          sort: 'asc' //i know, but that is still the only way sorting works for me, @5.3.4
        }]
      },
      {
        header: () => <span>{this.props.selectedYear}</span>,
        columns: [{
          header: 'Apr',
          id: 'april',
          accessor: data => data.months && data.months.length > 0 && data.months[3].value,
          width: 70,
          sortable: false,
          render: ({index, value, row, aggregated}) => this.handleMonthValuesRender(index, value, row, aggregated, 3)
        },{
         // more columns 
        }]
      }
    ]

I tried removing all grouped headers but didn’t fix the issue.

Any help is much appreciated, maybe I have to check something different than the columns too, I don’t know.

EDIT: To be more specific, probably the problem is with my code and not react-table, so I would like to know what causes this error.