How to parse a json double list online into dart in flutter?

Here is my code:

class PortfolioHistory {
  List<int> timestamp;
  List<dynamic> equity;
  String timeframe;

  PortfolioHistory(
      {this.timestamp,
        this.equity,
        this.timeframe});

  factory PortfolioHistory.fromJson(final json) {
    return new PortfolioHistory(
      equity: json['equity'].cast<double>(),
      timestamp: json['timestamp'].cast<int>(),
      timeframe: json['timeframe'],
    );
  }

  Map<DateTime, double> createCoordinates() {
    Map<DateTime, double> data = {};
    if (equity != null) {
      int length = equity.length;
      for(int i = 0; i < length; i++) {
        int milliEpoch = timestamp.elementAt(i);
        // coordinates
        DateTime x = DateTime.fromMillisecondsSinceEpoch(milliEpoch * 1000);
        data[x] = equity.elementAt(i);
      }
    }
    return data;
  }

}

Apparently this line data[x] = equity.elementAt(i); produces this error:

type 'int' is not a subtype of type 'double' in type cast

I declare my equity as a dynamic list as it takes data from a realtime database and sometimes, some elements of the list will be 'null'. Can anyone please help me?