NoSuchMethodError: Class 'RuntimeResponse' has no instance method 'json'.

NoSuchMethodError: Class 'RuntimeResponse' has no instance method 'json'.
Receiver: Instance of 'RuntimeResponse'
Tried calling: json(_Map len:0, headers: _ConstMap len:3)
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:38)
#1      _objectNoSuchMethod (dart:core-patch/object_patch.dart:85)
#2      main (package:starter_template/main.dart:24)
#3      action.<anonymous closure> (file:///usr/local/server/src/server.dart:116)
#4      _rootRun (dart:async/zone.dart:1399)
#5      _CustomZone.run (dart:async/zone.dart:1301)
#6      _runZoned (dart:async/zone.dart:1804)
#7      runZoned (dart:async/zone.dart:1747)
#8      action (file:///usr/local/server/src/server.dart:111)
<asynchronous suspension>
#9      main.<anonymous closure> (file:///usr/local/server/src/server.dart:185)
<asynchronous suspension>
#10     handleRequest (package:shelf/shelf_io.dart:138)
<asynchronous suspension>

const corsHeaders = {
  'Access-Control-Allow-Origin': 'berthou.github.io',
  'Access-Control-Allow-Methods': 'POST, GET, OPTIONS',
  'Access-Control-Allow-Headers': 'Content-Type, Authorization',
};

Future<dynamic> main(final context) async {
  
...

  if (context.req.method == 'OPTIONS') {
    return context.res.json(// --------------- EXCEPTION HERE ----------
      {},
      headers: corsHeaders,
    );
  }

  final request = {
    'api_key': octopusApiKey,
    'email_address': emailaddress,
  };

  final response = await http.post(...);

  if (response.statusCode == 200) {
    return context.res.json(
      {},
      headers: corsHeaders,
    );
  } else {
    return context.res.json(
      {},
      headers: corsHeaders,
    );
  }
}