"TypeError: Cannot set properties of undefined (setting 'flag')"

[BadIdeaException]

const polyk = require("polyk")

const vertices = [ 1,  1,
  5,  5,
  5,  0,
  4,  2 ]

const [ s1, s2 ] = [ 0, 2 ]
const [ e1, e2 ] = [ 9, 2 ] 
polyk.Slice(vertices, s1, s2, e1,e2)

results in TypeError: Cannot set properties of undefined (setting 'flag') in index.js line 241.

[DVLP]

Same problem, pops up at random