FormatException: Invalid port (at character 16)

[paulobreim]

Here is my code:
HttpServer.bind(InternetAddress.anyIPv4, port).then((server) {
print('Servindo em ${server.address}:${server.port}');
server.listen((HttpRequest request) {
handleGetRequest(request); // check de URL
request.response
..close();
});
});
I don't know why, but some times, the program stop with:
FormatException: Invalid port (at character 16)
http://0.0.0.0:81login.cgi
Sure that I am using port 81, but the port does not import.
This error occur even if there are anybody connecting.

Debuging the app I can see that the program is waiting for a connection in the line
server.listen((HttpRequest request)

From time to time, simply gives the message without anyone connecting

Another fact is that I am not put
import 'package:http_server/http_server.dart';
because when I put, I receive message that Target of URI doesn't exist