"Error: <path> attribute d: Expected number, "MNaN,NaNLNaN,NaNL…". " Error with D3

I'm importing some data from Quandl's API to make a chart of brent oil prices over the years. I'm serving the data from a HTTP-request made with Angular. Somehow the data being served is not being read as numbers, since i get the following error:

Error: attribute d: Expected number, "MNaN,NaNLNaN,NaNL…".

app.service('oilService', function($http) {
  delete $http.defaults.headers.common['X-Requested-With'];
  this.getData = function() {
  var URL = "https://www.quandl.com/api/v1/datasets/CHRIS/ICE_B1.json";
  return $http({
     method: 'GET',
     url: URL
  })
 }
})

.directive('oilGraph', function() {
 return {
  scope: {},
  controller: function(oilService) {
  var width = 200;
  var height = 100;

  var parseTime = d3.timeParse("%y-%m-%d");

  var x = d3.scaleTime().range([0, width]);
  var y = d3.scaleLinear().range([height, 0]);

  var valueline = d3.line()
      .x(function(d) { return x(d.date); })
      .y(function(d) { return y(d.settle); });
  var svg = d3.select("#oilgraph").append("svg")
        .attr("width", width)
        .attr("height", height);

  oilService.getData()
  .then(function(data) {

      var mapped_data = data.data.data.map(function(d) {
        return {
          date: parseTime(d[0]),
          settle: d[4]
        };
      })

      x.domain(d3.extent(data, function(d) { return d.date; }));
      y.domain([0, d3.max(data, function(d) { return d.settle; })]);

      svg.append("path")
          .data([mapped_data])
          .attr("class", "line")
          .attr("d", valueline);

      svg.append("g")
          .attr("transform", "translate(0," + height + ")")
          .call(d3.axisBottom(x));

      svg.append("g")
          .call(d3.axisLeft(y));
    })
   })
  }
 }
})