WebHooks and Processes with system action


I’ve tried the new webhooks API. All is working fine for creating a process with prefilled datas.

But if the first action of a process is a system action (in my case GetUsersFromDir), the process starts but the first action fails with the following error:

    CODE: ErrorWebServiceCall (#217)
SOURCE: WebServiceExecute
MESSAGE: Error while invoking the web method GetUsers : Exception has been thrown by the target of an invocation. : System.Web.Services.Protocols.SoapException: GetUsersFromDir Error: Object reference not set to an instance of an object.
   at Advantys.Workflow.Applications.Web.Services.GetUsersFromDir.GetUsersFromDirService.GetUsers(String WFGEN_CONTEXT)

Is there any special parameters to launch a process starting with a system action ?