Flutter mockito post always return null - flutter

Question

I'm trying to test my code that makes a post to login from an API using a mocked http client, but instead of returning what I asked for, it returns null, I did the same test but changing the endpoint and method to GET and it worked perfectly. I'm currently using flutter's http to make the requests, but I've already tested it with Dio and the result was the same, below is my code
Future<String> signIn(String email, String password) async {
final Map<String, dynamic> body = {"email": email, "password": password};
final String url = url_base + Urls.auth_login;
final Map<String, String> customHeader = {
"Content-type": "application/json",
};
String returnCode;
try {
var x = jsonEncode(body);
http.Response response = await client.post(Uri.parse(url), body: x, headers: customHeader);
var parsedJson = json.decode(response.data);
if (parsedJson.containsKey("token")) {
returnCode = parsedJson["token"];
} else {
returnCode = parsedJson["non_field_errors"][0];
}
}catch (e) {
throw ServerException();
}
if (returnCode == null) {
throw ServerException();
} else {
return returnCode;
}
}
and the test case:
class ClientMock extends Mock implements http.Client {}
void main() {
RemoteData remoteData;
group('Test signIn', () {
test('Login with email and wrong password', () async {
final clientMock = ClientMock();
remoteData = RemoteData(client: clientMock);
String jsonMockResponse =
'{non_field_errors: [Unable to log in with provided credentials.]}';
when(clientMock.post(any))
.thenAnswer((_) async => http.Response(jsonMockResponse, 400));
String loginReturn =
await remoteData.signIn('test#email.com', 'password123');
expect(loginReturn,throwsA(const TypeMatcher<ServerException>()));
});
}
I've already tested some things like changing 'any' for exactly the same thing the real function gets and it didn't work either.
The actual test return 'Instance of 'ServerException'', an in debug mode i could see that the return is null, and the last if is the one who throws this exception.