d3.v5.min.js:2 Error: <path> attribute d: Expected number, code example

Example: attribute d: Expected number, "MNaN

d3.csv('Book1.csv', convert, init); //load data, convert, then initialize chart

function convert(d) {
  return {
    date: new Date(d.date),
    value: +d.value         // convert string to number
  };
}