GetActivityInstanceList by ProcessInstanceId

I’m trying to get a list of Open activities for a specific Process. When I pass in ProcessInstanceId I get “runtime is unable to evaluate this expression”.

XmlNode resultNode = null;
ActivityInstanceListHeader myActivityInstanceListHeader = null;
// Settings and parameters
myActivityInstanceListHeader = new ActivityInstanceListHeader();
myActivityInstanceListHeader.Template = “ProcessesRuntime.ActivityInstanceListRss.txt”;
myActivityInstanceListHeader.Size = 1;
myActivityInstanceListHeader.Page = 1;
myActivityInstanceListHeader.ProcessInstanceId = 191900;

// Set NetworkCredentials with the credentials of the current connected user
workflowApi.ActivityInstanceListHeaderValue = myActivityInstanceListHeader;
workflowApi.Credentials = CredentialCache.DefaultCredentials;
// Call the Web Service API method
resultNode = workflowApi.GetActivityInstanceList(“Todo”);

Any ideas?