calling expectsJson() before body() of a post request breaks response parsing

[lukemim]

$response = \Httpful\Request::post($url)
->expectsJson()
->body(['abc'=>123], \Httpful\Mime::FORM)
->send();

results in incorrect response body parsing, for example:
Array (size=1) '{"exampleResponseFromServer":"data processed successfully"}' => string '' (length=0)
(the entire raw response body is put in an array key and the array value is empty string)

When ->expectsJson() is called after ->body() the response JSON is parsed correctly.