Error calling GetSigningUrl()

Hi, I am trying to get a signing url for a package and get this error: Could not get a signing url. Exception: Unexpected character encountered while parsing value: . Path '', line 0, position 0. Unexpected character encountered while parsing value: . Path '', line 0, position 0. at Silanis.ESL.SDK.Services.PackageService.GetSigningUrl(PackageId packageId, Role role) at Silanis.ESL.SDK.Services.PackageService.GetSigningUrl(PackageId packageId, String signerId) My code is very simple

EslClient client = new EslClient(_apiKey, _apiUri);
string url = client.PackageService.GetSigningUrl(new PackageId(packageId), signerId);

My package is created with a single signer that has a custom id.