hilite.replace is not a function

[swedenismycity]

when running the demo.html provided in the repository this board seems to return an error:

  Chessground(document.getElementById('board-4'), {
    orientation: 'black',
    coordinatesOnSquares: true,
    ranksPosition: 'left',
  });

the first 3 render just fine, but the 4th returns this error:

• Uncaught TypeError: hilite.replace is not a function

• hiliteOf svg.ts:396

• syncDefs svg.ts:103

• renderSvg svg.ts:91

• redrawNow chessground.ts:30

• redrawAll chessground.ts:47

• Chessground chessground.ts:54

• demo.html:38