cURL error 3: <url> malformed

[Edwinws]

Hi, I'm getting this error:

cURL error 3: <url> malformed
#0 /home/edwin/Documents/laravel/vendor/guzzlehttp/guzzle/src/RequestFsm.php(104): GuzzleHttp\Exception\RequestException::wrapException(Object(GuzzleHttp\Message\Request), Object(GuzzleHttp\Ring\Exception\RingException))
#1 /home/edwin/Documents/laravel/vendor/guzzlehttp/guzzle/src/RequestFsm.php(132): GuzzleHttp\RequestFsm->__invoke(Object(GuzzleHttp\Transaction))
#2 /home/edwin/Documents/laravel/vendor/react/promise/src/FulfilledPromise.php(25): GuzzleHttp\RequestFsm->GuzzleHttp\{closure}(Array)
#3 /home/edwin/Documents/laravel/vendor/guzzlehttp/ringphp/src/Future/CompletedFutureValue.php(55): React\Promise\FulfilledPromise->then(Object(Closure), NULL, NULL)
#4 /home/edwin/Documents/laravel/vendor/guzzlehttp/guzzle/src/Message/FutureResponse.php(43): GuzzleHttp\Ring\Future\CompletedFutureValue->then(Object(Closure), NULL, NULL)
#5 /home/edwin/Documents/laravel/vendor/guzzlehttp/guzzle/src/RequestFsm.php(135): GuzzleHttp\Message\FutureResponse::proxy(Object(GuzzleHttp\Ring\Future\CompletedFutureArray), Object(Closure))
#6 /home/edwin/Documents/laravel/vendor/guzzlehttp/guzzle/src/Client.php(165): GuzzleHttp\RequestFsm->__invoke(Object(GuzzleHttp\Transaction))
#7 /home/edwin/Documents/laravel/vendor/guzzlehttp/guzzle/src/Client.php(150): GuzzleHttp\Client->send(Object(GuzzleHttp\Message\Request))
#8 /home/edwin/Documents/laravel/vendor/maknz/slack/src/Client.php(357): GuzzleHttp\Client->post(Array, Array)
#9 /home/edwin/Documents/laravel/vendor/maknz/slack/src/Message.php(410): Maknz\Slack\Client->sendMessage(Object(Maknz\Slack\Message))

I get this error after running
Slack::to('#jarvis-testing')->send('testing');

I've set the endpoint field in /config/packages/maknz/slack/config.php, while the rest of the fields are default.

Interestingly, the other package (https://github.com/maknz/slack) works fine.

I'm running on laravel 4.2