Multi-sheet conditional formatting crashes when not all sheets have rules

Two sheets, conditional formatting rules only on the first sheet. Passing an empty array [] for the second sheet works, but undefined or omitting it crashes.

node -e '
  var w = require("write-excel-file/node")
  var cf = new Array(2)
  cf[0] = [{
    cellRange: {from:{row:1,column:1}, to:{row:1,column:1}},
    condition: {operator:"=", value:"a"},
    style: {backgroundColor:"#ff0000"}
  }]
  w([[[{value:"a",type:String}]], [[{value:"b",type:String}]]],
    {sheets:["S1","S2"], conditionalFormatting:cf, filePath:"/tmp/b.xlsx"}
  ).catch(function(e) { console.log(e.message) }) '

TypeError: Cannot read properties of undefined (reading "length")'