Worklist Criteria Filters

Can someone maybe explain to me why the following code returns an empty worklist (IE exception on return worklist[0])

var crit = new WorklistCriteria();
crit.AddFilterField(WCLogical.And, WCField.ProcessData, "Activity Id", WCCompare.Equal, id);
Worklist worklist;
connection.Open(ServiceConfiguration["K2ServerName"].ToString());
worklist = connection.OpenWorklist(crit);
return worklist[0];

BUT, The following code returns the correct worklist task:

 var crit = new WorklistCriteria();
            //crit.AddFilterField(WCLogical.And, WCField.ProcessData, "Activity Id", WCCompare.Equal, id);

            Worklist worklist;
            connection.Open(ServiceConfiguration["K2ServerName"].ToString());
            worklist = connection.OpenWorklist(crit);

            foreach (WorklistItem task in worklist)
            {
                var processData = task.ProcessInstance.DataFields;

                foreach (DataField field in processData)
                {
                    var name = field.Name;
                    if (name == "Activity Id")
                    {
                        if (field.ValueType == DataType.TypeInteger)
                        {
                            var value = Convert.ToInt32(field.Value);
                            if (value == id)
                            {
                                return task;
                            }
                        }
                    }
                }
            }